I did not write this guide. Credit actually goes to Seerinox on Tichondrius.
World of Warcraft - English (NA) Forums -> Achievements Guide

I just thought it was a good guide for achievements and it should be brought to everyones attention. Good luck getting your achievements

Achievements Guide:

How to use this guide: It's big and it's not in alphabetical order (the order is the one I see on my achievements screen if you're curious). This would take me a long time to rectify so I won't do it yet, but until then you can use your browser's search tool in order to find the one you need. If there's a mistake in an achievement name please let me know.

Special Thanks: I'd like to thank everyone who helped me complete my achievements. I'd like to thank Painreaver for walking me through some of them. Most importantly, I'd like to thank Wowhead staff and users for the awesome website and the great comments you find there.

General:

Tastes Like Chicken / It's Happy Hour Somewhere:
You can most if not all of this in Shattrath and capital cities by going to different vendors (example: Meat / Fish / Mushrooms, etc...) If that's not enough hit the AH and buy some food there or cook your own. Don't worry if you already completed some of this and can't remember what you ate and drank, there's more than enough out there for you to complete this easily. Mage food also counts towards those.

25 Tabards:
Here's a list of the tabards that work for this: Represent - Achievement - World of Warcraft
In WotLK an NPC will also allow you to buy back all the tabards you got from Reputations and will restore any quest ones you might have deleted. Others like the Horde ones are not restored since you can get them again if you destroy them.

Leading The Cavalry:
The easiest and cheapest way to do this is to get exalted with the five horde races as well as with netherwing or skyguards. Together, these 7 reputations will sell over 25 mounts and you can also buy Horde flying mounts (not a reputation). Get those that work best for you! And no, AQ mounts don't count unless you have the quest line one.

Taurens are handicaped for this achievement. Here is the list of mounts they can learn:
5 Kodos (TB)
6 Wolves (Org)
3 PvP Mounts
8 Talbuk (Mag'har)
2 Talbuk (Halla)
1 Wolf (Alterac Valley)
6 Netherwing Drakes (Netherwing)
5 Netherrays (Skyguard)
7 Wind Riders (Shadowmoon)
4 Ahn'Qiraj bugmounts
Cenarion Expedition Mount

As you can see that is only 48 mounts, taurens have to wait until WotLK or have to get rare mounts in order to complete this achievement pre-TBC (midnight, deathcharger, anzu, ZG mounts). Unless they're an engineer.

Well Read:
I recommend using Venator's Guide on Wowhead at: Well Read - Achievement - World of Warcraft
Venartor's guide is the 9th comment on the page. Due to lack of space and the time the author took to making his guide I would rather link it than repost it, a lot of work was obviously involved.

Shop Smart, Shop Pet... Smart
The first thing you have to do is visit your faction's pet vendors.

Both Factions:
-Magus Tirth (One Thousand Needles)
-Narkk (Booty Bay)
-Dealer Rashaad (Stormspire)
-Flik (Darkmoon Faire)

Horde:
-Xan'tish (Orgrimmar)
-Jilane (Eversong Woods)
-Halpa (Thunder Bluff)

Alliance:
-Sixx (Exodar)
-Donni Anthania (Elwynn Forest)
-Lil Timmy (Stormwind, Rare)
-Shylenai (Darnassus)
-Yarlyn Amberstill (Dun Morogh)
Another piece of advice is to regularly check both the Horde and the Neutral AH, go in the Pets tab and enable "Usable Items only"
You should still be short with all these but not by much. Go on Wowhead and look at the pets list and check which ones you don't have and go for the easiest ones at this point.

Going Down? (Thanks Laif for pointing out the Iceblock)
Jump from the Scyer's tier and land next to the ramp that's at the bottom. You have to land on the floor where all the banks and portals are. Make sure you're at full health. Don't use Levitate or Slowfall or any such ability or item either, Divine Shield and Ice Block do work though.

The Keymaster:
Auchenai Key - Be Honored with Lower City
Crescent key - in DM east, the small imp you have to chase drops it, Pusilin
Key of Time - Be Honored with Keepers of Time
Stratholme Key - drops from Magistrate Barthilas, only one person may loot it
Shadow Labyrinth Key - Sethekk Halls final boss, Talon King Ikiss
Shadowforge Key - Reward for the last part of Dark Iron Legacy, a two quests chain that starts from Franclorn Forgewright, a ghost which you can only see if you are dead, he is in Blackrock mountain and you will spot him as you go down to BRD if dead, tracking low-level quests while dead might help you find him, but I'm not sure tracking works while dead
Shattered Halls Key - Reward for the last part of a four quests chain that starts when you kill Smith Gorlunk outside BT and loot his Primed Key Mold. The last part requires you to kill a Fel Reaver
Scarlet Key - SM Library, Arcanist Doan's small chest at the back of the room
Skeleton Key - Reward from a six quest long chain that starts at High Executor Derrington who can be found in Tirisfal Glades at the Bulwark
Workshop Key - Drops from Electrocutioner 6000 in Gnomeregan
The Violet Hold Key: WotLK item you get in Dalaran when getting the first quest to go into Violet Hold

To All The Squirrels I've Loved Before: (credit for the list goes to Kattiara17 from Wowhead, I edited it to remove Artic Hare and add Steam Frog and Borean Frog locations)
Personal tip: If you have trouble finding a critter, look up it's potential locations on wowhead and make a /target macro you can spam while running around the zone.

Squirrel: Darnassus, Ashenvale, Dire Maul, Magisters' Terrace, Zul'Aman
Steam Frog: Borean Tundra (WotLK)
Borean Frog: Borean Tundra (WotLK)
Cat: Eversong Woods, Silvermoon City, Elwynn Forest
Chicken: Westfall, Hillsbrad Foothills, Elwynn Forest, Arathi Basin, Duskwood, Tirisfal Glades, Shattrath City, The Barrens, Azuremyst Isle, Dustwallow Marsh, Redridge Mountains
Cow: Elwynn Forest, Hillsbrad Foothills, Arathi Highlands, Westfall
Crab: The Steamvault
Deer: Elwynn Forest, Darnassus, Teldrassil, Moonglade, Ashenvale, Blade's Edge Mountains, Alterac Mountains, Azuremyst Isle, Darkshore, Stonetalon Mountains, Western Plaguelands, Westfall
Ewe: Shattrath City (Under the north bridge, use /tar ewe to find it)
Fawn: Elwynn Forest
Frog: Maraudon, The Slave Pens,The Steamvault
Gazelle: The Barrens
Hare: Durotar
Parrot: Un'Goro Crater
Rabbit: Dun Morogh, Elwynn Forest, Scarlet Monastery
Ram: Loch Modan, Wetlands
Sheep: Elwynn Forest, Redridge Mountains, Loch Modan, Hillsbrad Foothills, Alterac Mountains, Westfall
Skunk: Terokkar Forest, Bloodmyst Isle, Azuremyst Isle
Swine: Durotar, The Barrens
Toad: Black Temple, The Steamvault, The Underbog
Prairie Dog: The Barrens, Nagrand, Mulgore, Arathi Highlands, Westfall

Friend or Fowl:
This will be in WotLK in Borean Tundra at the Human Keep.

Quests:

Loremaster:
You have to go everywhere around the world and complete any quest you find. Here are some tips:
-Track low-level quests
-Make sure your log is never full
-Don't skip any quest
-If you did all the zones in the old world and are short go on wowhead, look up the zone, and select "Show Horde Questgivers" in the drop down menu above the map.
-Look on Wowhead for a list of items that give quests

Blade's Edge Bomberman:
Go to the Burning Legion camp north of O'grila for this, and make sure you don't lose a single second of bomb uptime. Don't try to avoid other players, if you fail a throw just reset it, you lost too much time. Make sure your 15th bomb is thrown at the southern-most cannon so that you can fly back and turn in ASAP (you have to turn in for credit). Also try to avoid being clumsy and losing time turning the quest in (in might sound stupid, but this achievement is hard and every second really counts).

Bombs Away:
This one is easier, but has random factors that are hard to prevent. First are the kaliris, if you aggro one and get dazed or hamstrung you lose, so try to avoid them. The other one is the respawn of the eggs, try to wait a few minutes next to the quest giver and go a few minutes after you saw someone take the quest if it's an issue.
The route I chose was the following one: Maps - World of Warcraft

Exploration:
Medium Rare: (Credit goes to joshuamclark from Wowhead for the macro)
Make the following macro and spam it as found fly around Outlands:
/tar Fulgor
/tar Mekt
/tar Vorak
/tar Martic
/tar Bog Lurk
/tar Coilfang Em
/tar Crip
/tar Doomsay
/tar Okrek
/tar Goretoo
/tar Voidhunt
/tar Morc
/tar Hema
/tar Speak
/tar Nura
/tar Ever-Core
/tar Chief Engineer L
/tar Collid
/tar Kraat
/tar Ambassador J

Dungeons & Raids:
Leeeeeeeeeeeeeroy! (I did count the Es!)
Take a few (6 or more should be fine if every class can aoe, but be safe) friends with AoE with you to UBRS in Blackrock Mountain, make sure one of them has the key. Clear all the way to Emberseer and then make sure everyone is ready before you run into the next room and walk over all the eggs you see while doing your best AoE damage possible..

Heroic Dungeons:
Completing a Heroic gives credit for both the Heroic and Normal Achievement.

Professions:
Cooking:
Chef de Cuisine:
Make sure you check the AH for recipes you don't have (Use the "Usable Items only" checkbox). Every recipe is either taught by every cooking master, learned from an object (look up the ones you don't have on Wowhead), or taught by the Barmaid in the Shattrath Inn. I also recommend getting the Addon Ackis Recipe List in order to know what recipes you're missing and where to obtain them.

The Outland Gourmet:
You must know every recipe taught in Outland, 90% can be bought, 3 are taught in Shattrath's main Inn, and the others are from the level 70 cooking dailies.

Fishing:
The Scavenger: (Credit goes to sonic314 on Wowhead for making the list and Firehead on this thread for adding a Floating Wreckage zone)
Steam Pump Flotsam (Zangarmarsh)
Bloodsail Wreckage (Coastal STV)
Schooner Wreckage (Hillsbrad Foothills, Wetlands)
Waterlogged Wreckage (STV River, Theramore Beach)
Floating Wreckage (Feralas, Tanaris)

Outland Angler: (Credit goes to Tadzulempke on Wowhead for making the list)
Brackish Mixed School - Terokkar Forest/Zangarmarsh
Bluefish School - Nagrand
Mudfish School - Nagrand
School of Darter - Terokkar Forest
Highland Mixed School - Terokkar Forest (only reachable with flying mounts)
Sporefish School - Zangarmarsh

The Lurker Above:
You need to be an engineer or Revered with the Skyguards for this one, unless you are a mage or a priest. Make a raid and go into SSC, jump to the water, use your cloak (skyguard one or parachute), and have a shaman cast Water Walking on you or use an elixir of Water Walking. Walk to the middle of the cavern, fish, die, win!
Priests: Just levitate, no fall damage and water walking.

Deadliest Catch:
Complete the quest Nat's Measuring Tape if you haven't, it is found in ZG where you summon the fish boss. Buy a Mudskunk Lure from Nat Pagle in Dustwallow Marsh after having turned in the quest. Fish five Mudskunks in ZG and then use the item at the camp.

Old Crafty / Old Ironjaw:
It is completely random and expect to fish up roughly 600 useless ones before you find each of them.

Mr. Pinchy's Magical Crawdad Box:
Very low drop from Highland Mixed Schools found in high places (those you can only reach with a flying mount: west of Garadar, east of Alliance Keep, and Skettis). Not only does Mr. Pinchy have a very low drop but you also aren't guaranteed he'll give you the Box in one of the three summons.

Fish Don't Leave Footprints:
You have a random chance to fish up the book that teaches this skill when fishing.

The Fishing Diplomat
If you are Horde jump into the water to the right when entering Stormwind and fish on the banks there.
If you are Alliance, fish in the Valley of Spirits and enter Orgrimmar from the West entrance (Barrens)

One That Didn't Get Away (Credit goes to Ellwynd on Wowhead for the list)
103 Pound Mightfish - Bay of Storms in Azshara
15 Pound Mud Snapper - Inland lakes in starting areas
29 Pound Salmon - ?
52 Pound Redgill - ?
Baby Coralshell Turtle - Northrend
Steelscale Crushfish - Hilsbrad Foothills?
113 Pound Swordfish - Northrend
22 Pound Lobster - ?
32 Pound Catfish - Desolace
68 Pound Grouper - Ferelas
Rockhide Strongfish - Stranglethorn Fishing Extravaganza

Note: These locations are places they have been caught they may also be found in additional areas. They are very rare, be patient.

Reputations:

Ambassador of the Horde:
There are several ways to do these. AV turn-ins give rep with each faction but goes up rather slowly. Questing gives rep for every faction, but it's slower (if you want to do Loremaster achievements though do them before this one). Turning in Runecloth gives 75 Reputation per 20 Runecloth and is the fastest, but it's only worth it if you only have one or two reputations you want to level. If you haven't completed every low level quest in the game you should do that first, as it is the easiest way to get reputation.

Classic:
The Argent Dawn:
Do their quests, that's a good start. The best way after this is to farm Undead Stratholme or Scholomance bosses (I recommend Stratholme for the Deathcharger, which gives a Feat of Strength, is cool, and has a one percent drop rate now) with your Argent Dawn trinket in order to get Scourgestones, you get it from a quest at the Bulwark. It's rather difficult at 70 though.

Brood of Nozdormu:
Get the quest to kill BWL's second boss: Broodlord in Cenarion Hold, Silithus. Complete it, and then keep farming silithids for reputation, or run AQ40 (way faster but not soloable).

Hydraxian Waterlords:
Run MC. They also give a handful of MC quests if you go to Azshara at 79, 73.

Hero of the Zandalar Tribe:
Run ZG and save coins / bijous, or buy them off the AH and turn them in.

They Love me in that Tunnel:
Go to Felwood and make sure you complete the quests given by the neutral furbolg at the entrance to the tunnel (at the Northeast corner of the zone) and at the exit (Winterspring side). Once you've done that you can start farming the furbolgs west of the tunnel, you get reputation for kills and for turning in feathers they drop.

The Burning Crusade:
Skyshattered:
This is a race, but you don't have to finish first, simply finishing is enough. Stay clear of him to have time to see the fireballs coming (like in BEM bombing), cut corners, be careful because he sometimes turns around, don't get too far, and don't give up. It's hard but you'll get there.

The Violet Eye:
Run Karazhan. Doing the quests they give outside Karazhan is a great boost too.

Shattrath Divided:
Complete the faction's quests (found scattered across Outlands) and then turn their faction-specific items (if you don't know what they are look at the repeatable quests on the faction's tier)

The Czar of Sporegarr:
Do their quests to have a nice boost, they are east of the Spawning Glen and in Sporegarr Village, both in Zangarmarsh. The fastest way is to have a rogue and stealth UB to get every sanguine hibiscus in the zone. If you don't have a rogue you can buy them. You can also farm nagas and complete a repeatable quest given in Sporeggar Village for reputation, but it is slower.

Chief Exalted Officer:
There are many ways to do this one. Ogre warbeads in Nagrand is a good one. Another is to kill Etherals in Netherstorm who drop insignas. Mana-Tombs give reputation for this faction too, and daily heroic and dungeon quests give some too. The consortium also has quests you should complete in order to get a nice boost, but they are scattered all over Outlands.

Mag'har of Draenor:
Complete all their quests found in Garadar (Nagrand) and in Mag'har post (Hellfire Peninsula), and farm ogres in the South-East Village of Nagrand. I recommend saving the beads they drop for Consortium, but if don't want to they're a huge boost to Mag'har reputation (50 rep each).

On Wings of Nether:
Once you have your epic mount do the chain quest that starts from the blood elf hunter east of the Sanctum of Stars in SMV. Once you're neutral go on Netherwing Ledge and do the dailies there. Pick up any egg you find on the isle, they are a great boost to reputation

You're so Offensive:
Complete SSO dailies on the Isle of Quel'Danas (a portal in Shattrath leads to it) and run Magister's Terrace (Heroic gives more reputation).

A Quest a Day Keeps the Ogres at Bay:
Complete the chain quest you find at the BEM arena (the questgiver is on top of the tower east of it). You can then do O'grila and Skyguard (BEM ones only) dailies until Exhalted, they are found in O'grila and in the skyguard post north of the village.

Flying High Over Skettis:
Complete the quest chain found next to Nutral the flight-master in Shattrath. Then do Skettis (Terokkar Forest) and Blade's Edge Mountains Skyguard dailies (north of O'grila).

The Burning Crusader:
For each of these factions do the quests found in their twon and then run their instances (heroic will give more reputation).
Thrallmar (Hellfire Peninsula): Ramparts, Blood Furnace, Shattered Halls
Lower City: Auchenai Crypts, Sethekk Halls, Shadow Labyrinth
The Sha'tar: Mechanar, Botanica, Arcatraz
Cenarion Expedition (Zangarmarsh): Slave Pens, Underbog, Steamvaults
Keepers of Time (Tanaris): Escape from Durnholde Keep, Black Morass

The Scales of Sand:
Run Mount Hyjal repeatedly.

Sworn to the Deathsworn:
Run Black Temple repeatedly.

Cenarion War Hyppogryph:
You can buy this item in the Cenarion Village of Zangarmarsh once exalted with Cenarion Expedition, it costs 2000 gold.

World Events:

Hallow's End:

Sinister Calling:
These items are either found in Goody Bags from Trick or Treating in inns, or by killing the Headless Horseman.

Check your Head:
Weighted Jack-o'-Lanterns drop from the Headless Horseman or from Goody Bags you get from Trick or Treating in inns.

The Masquerade:
Wands are found in Goody Bags from Trick or Treating in inns. They can only be used on party/raid members, and not on self. The easiest way to do this is to ask in the Trade channel and pay for it.

Rotten Hallow:
The quests are found west of Undercity (left on the road from Undercity to The Sepulcher). They are completed in Southshore (one requires you to drop rotten eggs in the middle of the village, the other to go in the inn and taint the barrel on the ground floor). You turn them in to the quest giver west of Undercity.

The Mask Task:
Masks are found in Goody Bags from Trick or Treating in Inns

Out With It:
Eat Tricky Treats until you get the debuff Upset Tummy. Tricky Treats appear in your bags if you are near the Headless Horseman in SM Graveyard upon his death, stay around him for more to appear.

Bring Me The Head of... Oh Wait:
Kill the Headless Horseman, summoned in SM Graveyard by a daily quest you find there.

The Savior of Hallow's End:
Complete the daily at one of your faction's starting villages (Razor Hill, Brill, Falconwing Square and Bloodhoof Village for the horde). The event happens every fifteen minutes.

That Sparkling Smile:
Get a Toothpick from the Goody Bags obtained when Trick or Treating in an Inn and use it. They have a good drop rate and you shouldn't need too long to find one.

Trick or Treat!:
Talk to an Innkeeper, select Trick or Treat.

G.N.E.R.D. Rage:
G.N.E.R.D.s are found in Candy Buckets in inns. They now last 14 days but if you run out you can get some more on an alt and mail them to your character since they are not soulbound. You can also buy them from other players.

A Mask for All Occasions:
Comment: This will not be required in order to obtain the title "The Hallowed" anymore in patch 3.0.3.