Twenty-Five Tabards

Here is a concise list of tabards currently in game. Of course you can always look them up here, but I thought it would be useful to have them all classified by source.

Reputation factions in Burning Crusade (12):

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• Aldor Tabard / Scryers Tabard
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• Cenarion Expedition Tabard
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• Consortium Tabard
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• Honor Hold Tabard / Thrallmar Tabard
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• Keepers of Time Tabard
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• Kurenai Tabard / Mag'har Tabard
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• Lower City Tabard
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• Ogri'la Tabard
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• Sha'tar Tabard
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• Skyguard Tabard
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• Sporeggar Tabard
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• Tabard of the Shattered Sun
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Reputation factions in Wrath of the Lich King (4):

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• Tabard of the Argent Crusade
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• Tabard of the Ebon Blade
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• Tabard of the Kirin Tor
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• Tabard of the Wyrmcrest Accord
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Argent Tournament (7):

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• Argent Crusader's Tabard
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• Silver Covenant Tabard / Sunreaver Tabard
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• Exodar Tabard / Silvermoon City Tabard
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• Ironforge Tabard / Thunder Bluff Tabard
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• Stormwind Tabard / Orgrimmar Tabard
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• Gnomeregan Tabard / Sen'jin Tabard
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• Darnassus Tabard / Undercity Tabard
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Other sources (5):

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• Tabard of Summer Flames / Tabard of Summer Skies — reward from Shard of Ahune quest during the Midsummer Festival.
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• Green Trophy Tabard of the Illidari / Purple Trophy Tabard of the Illidari — reward from Battle of the Crimson Watch quest.
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• Tabard of the Scarlet Crusade — drops in Scarlet Monastery.
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• Tabard of the Hand / Blood Knight Tabard — quest reward from The Unwritten Prophecy (draenei only), or True Masters of the Light (blood elves only).
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• Guild Tabard — what can I say…
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PVP tabards (7):

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• Arathor Battle Tabard / Battle Tabard of the Defilers — rewarded for getting Exalted with League of Arathor / Defilers.
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• Knight's Colors / Stone Guard's Herald — can be purchased with 20 AV marks + 20 AB marks + 20 WSG marks.
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• Private's Tabard / Scout's Tabard — can be purchased with 3 AB marks + 3 WSG marks
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• Silverwing Battle Tabard / Warsong Battle Tabard — can be purchased with 60 WSG marks.
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• Stormpike Battle Tabard / Frostwolf Battle Tabard — can be purchased with 60 AV marks.
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• Furious Gladiator's Tabard : Relentless Gladiator's Tabard — requires 2350 arena rating in 3v3 or 5v5.
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• Tabard of Conquest / Tabard of Conquest — reward for Master of Isle of Conquest.
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Achievement awards (4):

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• Loremaster's Colors — Reward for The Loremaster.
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• Tabard of the Achiever — Reward for this achievement, Twenty-Five Tabards.
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• Tabard of the Explorer — Reward for Explore Northrend.
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• Tabard of Brute Force — Reward for Brutally Dedicated.
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Obtainable via Redemption (8):

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• Tabard of Brilliance
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• Tabard of Flame
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• Tabard of Frost
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• Tabard of Fury
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• Tabard of Nature
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• Tabard of the Arcane
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• Tabard of the Defender
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• Tabard of the Void
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No longer obtainable (5):

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• Competitor's Tabard — awarded for participation in 2008 Spirit of Competition event.
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• Contest Winner's Tabard — awarded to people with highest number of honorable kills on each server before the introduction of current honor system.
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• Tabard of Stormwind — was never available to players.
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• Tabard of the Argent Dawn — reward during the Scourge Invasion event.
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• Tabard of the Protector — quest reward during the Opening of Dark Portal event.
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NOTE: at the last edit there were 72 total tabards in wowhead database. Among those, 2 tabards do not count towards the achievement: Enchanted Illidari Tabard (conjured item) and Tabard of the Scarlet Crusade (not in game?).

EZ-mode way to get your shiny Tabard of the Achiever!
Collest and equip these 25 tabards to obtain it.

Horde / Alliance:
Note: the first 6 tabards are purchased from the factions quartermasters in their capital city.

1. 
   
2. Orgrimmar Tabard / Stormwind Tabard
3. 
   
   
4. Undercity Tabard / Darnassus Tabard
5. 
   
   
6. Darkspear Tabard / Ironforge Tabard
7. 
   
   
8. Thunder Bluff Tabard / Gnomeregan Tabard
9. 
   
   
10. Silvermoon City Tabard / Exodar Tabard
11. 
    
    
12. Bilgewater Cartel Tabard / Gilneas Tabard
    Note: the following tabards are purchased from the factions quartermasters in Northrend.
13. 
    
    
14. Tabard of the Argent Crusade
15. 
    
    
16. Tabard of the Kirin Tor
17. 
    
    
18. Tabard of the Wyrmrest Accord
19. 
    
    
20. Tabard of the Ebon Blade
    Note: these tabards are purchased from the factions quartermasters in Cataclysm Zones(80-85).
21. 
    
    
22. Tabard of Therazane
23. 
    
    
24. Tabard of the Guardians of Hyjal
25. 
    
    
26. Tabard of the Earthen Ring
27. 
    
    
28. Tabard of Ramkahen
29. 
    
    
30. Tabard of the Dragonmaw Clan / Tabard of the Wildhammer Clan
31. 
    
    
32. Hellscream's Reach Tabard / Baradin's Wardens Tabard Cost: 40 Tol Barad Commendations
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33. 
    
34. Guild Tabard Purchased from Tabard Vendors in any capital city.
35. 
    
    
36. Scout's Tabard / Private's Tabard Purchased from Tabard Vendors in any capital city. Cost: 50 honor points.
37. 
    
    
38. Stone Guard's Herald / Knight's Colors Purchased from Tabard Vendors in any capital city. Cost: 100 honor points.
39. 
    
    
40. Illustrious Guild Tabard Purchased from guild vendors at atleast friendly reputation. Note: this tabard is BoA and can be send between characters. Cost: 125g
41. 
    
    
42. Renowned Guild Tabard Purchased from guild vendors at atleast honored reputation. Note: this tabard is BoA and can be send between characters. Cost: 250g
43. 
    
    
44. Tabard of the Scarlet Crusade Drops off the Scarlet Trainees that spawn after killing Herod in Scarlet Monastery: Armory
45. 
    
    
46. Tabard of the Explorer Reward from completing Explore Northrend
47. 
    
    
48. Green/Purple Trophy Tabard of the Illidari. It is possible to change color of this tabard, this is how:
    Step One: Kill Val'Zareq the Conqueror(Who is located here.) and pick up The Quest Starter.
    Step Two: Start the quest and go here. Talk to this guy inside a crystal. Start the next quest and defend against four waves of demons.
    Step Three: Return to A'dal in Shattrath City. Complete the quest and you'll recieve your reward containing either a Green or Purple Trophy Tabard of the Illidari.
    Step Four: Equip the tabard, and then destroy it. Go to a tabard vendor(Horde/Alliance) She will have 2 new options, shown here. If you got the purple tabard first and destroyed it, select "I've lost my Green Trophy Tabard of the Illidari". If you got the green one, select purple.
    This way they'll count as two tabards.
49. 
    
    
50. Mentioned above.

I hope this guide helped you get your Precious.
BTW: If you have any corrections to this guide, or useful info, please leave it as a comment and I will do my best to improve the guide. :-)
EDIT: Added Tabard of Therazane thanks to dotorion.

According to Tigole as soon as the Achievement counts the tabard you can delete it or sell it back to the vendor.