Guild Tabard: Probably the first tabard you'll ever have. These can range from the fierce and frightening to the cute and cuddly. This tabard -- of Magtheridon's Nocturnal guild -- clearly went for the latter effect. You can also use IGN's Guild Tabard Generator to preplan your own tabard. You can get it at the tabard vendor in any major city.
Tabard of the Argent Dawn/Tabard of the Protector: These two tabards look the same, but one of them is for service to the Argent Dawn, while the other is for "heroism at the Battle of the Dark Portal." The regular tabard could be obtained during the Scourge Invasion event, by turning in Necrotic Runes. The Protector tabard was available by doing a quest to kill demons at the Dark Portal the week before Burning Crusade came out. If you get the Protector one, you can use it every five minutes to give off a bright light and /flex, "showing your heroism."
Green/Purple Trophy Tabards of the Illidari: These are the same one Illidan's minions in Shadowmoon Valley wear. You get them by going through a vicious quest chain in Shadowmoon Valley, which starts with a drop off the patrolling level 72 elite Val'Zareq the Conqueror on the Path of Conquest. You take the book you get from him and bring it to an imprisoned draenei on the Path, and he starts an event called the Battle of the Crimson Watch. Illidan sends four waves of many, many elite demons and blood elves at you. Bring a warlock, or possibly two separate groups. If you survive, Illidan challenges you to come face him at the Black Temple, and you get a nice ring and one of these two tabards from the Naaru in Shattrath City.
Tabard of the Scarlet Crusade: This beauty drops off the mobs that attack you after the Herod event in Scarlet Monastery. It's only a 0.4 percent drop, so get used to doing a LOT of runs if you want it.
Tabard of Flame: You can get this by getting a rare card in the WoW trading card game.
Tabard of Frost: You can also get this by getting a rare card in the WoW card game.
Contest Winner's Tabard: Sorry, you're too late to get this. Apparently it was given out as a reward by Blizzard to one Horde and one Alliance on each server who got the most honor kills in a set amount of time when the honor system was first released.
Private's Tabard/Scout's Tabard: You can get these for three Warsong Gulch marks of honor and three Arathi Basin marks of honor at your nearest PVP good outlet. Sorry, faction restrictions apply!
Knight's Colors/Stone Guard's Herald: Same thing, except you need twenty of each token, including Alterac Valley marks.
Tabard of the Hand: This comes from an Alliance level 20 quest on Bloodmyst Isle, one of the new Draenei zones. The quest, "The Unwritten Prophecy", starts with Demolitionist Legoso at Vindicator's Rest, involves blowing up things and killing demons, and ends with a big parade for you in Shattrath City. You get this tabard and your choice of rings from Prophet Velen.
Aldor Tabard: This tabard comes from getting exalted with the Aldor faction in Shattrath City.
Scryers Tabard: You can get this by getting exalted with the Scryer blood elves in Shattrath City.
Cenarion Expedition Tabard: This comes from exalted with the Cenarion Expedition in Zangarmarsh.
Consortium Tabard: You get this from being exalted with the Consortium in Netherstorm.
Honor Hold Tabard: This is available at exalted with Honor Hold in Hellfire Peninsula. Alliance only!
Thrallmar Tabard: The tabard is ready for purchase when you reach exalted with the Horde's Thrallmar in Hellfire Peninsula.
Kurenai Tabard: If you're exalted with the Kurenai on Alliance, this is your reward. Slightly less blurry in-game, I hope.
Mag'har Tabard: You get this for hitting exalted with the Mag'Har orcs on Horde side.
Lower City Tabard: This tabard is available at exalted with the Lower City exiles in Shattrath City.
Sha'Tar Tabard: This is buyable at exalted with the Sha'Tar naaru in Shattrath City.
Warsong Battle Tabard/Silverwing Battle Tabard: You can get these for sixty Warsong Gulch marks of honor at the entrance to WSG for each faction.
Frostwolf Battle Tabard/Stormpike Battle Tabard: Available inside Alterac Valley for sixty AV marks of honor.
Battle Tabard of the Defilers/Arathor Battle Tabard: Available for sixty Arathi Basin marks at the entrance to the battleground.
Blood Knight Tabard: Blood Elf paladins get this in addition to their epic mount when they finish their level 60 epic mount quest.
What tabard are you wearing? Which ones do you hope to wear in the future? Do you wish that casters looked less ridiculous in tabards, or what?
Thanks to Moist, Exiledchaos, Allakhazam, Wowhead, WoWWiki, and thottbot.com for the pictures provided here.