Tirion Fordring built Argent Tournament in Icecrown and invited excellent players from both Alliance and Horde to oppose the Lich King. They aim at choosing the most excellent players to fight the Lick King. 
For players, there are a lot more quests and dailies that can help quickly improve maincity reputation in their faction, and become Champion in every maincity. Players also will gain lots of WOW gold and Champion's Seals, they can buy pets, mounts, tabards, ilvl80 gears, heirlooms from the city vendor. One of the pets called Argent Squire/Argent Gruntling, He can be summoned for 3 minutes once every eight hours, and will then function either as a bank, a vendor or a mailbox.
Getting the Champion title from each main city is very easy, but you need much more Champion's Seal to buy the mounts and pets. Btw, pets also can be bought in AH, but mounts only available buying with Champion's Seal. There are 14 mounts you can buy, totally need around 1,000 Champion's Seal.
For mounts, pets junkies, Argent Tournament is a good choice! Meanwhile, it asks a comparatively long time.