The Wrath of the Lich King expansion added several new factions to World of Warcraft, and it has also given players a new method of earning reputation called "championing." Here are some pointers for getting reputation rewards in Northrend.
Alliance Vanguard - Rewards
The Alliance Vanguard is made up of four subfactions, each of which is connected to your Alliance Vanguard reputation. There are the Explorers' League, The Frostborn, The Silver Covenant, and the Valiance Expedition. There are no item rewards for the subfactions, but any reputation gained with a subfaction also increases your Alliance Vanguard reputation.
Horde Expedition - Rewards
The Horde Expedition also consists of 4 subfactions: The Hand of Vengeance, The Sunreavers, The Taunka, and the Warsong Offensive. The Horde Expedition is led by Garrosh Hellscream in Warsong Hold, and that's also where you find the Quartermaster. Earning reputation for any of the subfactions contributes to your overall Horde Expedition standing.
Kalu'ak - Rewards
This tribe of walrus-like tuskarr has two Quartermasters in Northrend. One is Sairuk at Moa'ki Harbor in Dragonblight, the other is Tanaika at Kamagua in Howling Fjord.
Sons of Hodir - Rewards
Based in Dun Niffelem in the Storm Peaks zone, the Sons of Hodir are frost giants that start out as aggressive to players. To gain neutral standing with them you need to do the quest chain that begins with "They Took Our Men!" in K3. Most of the rewards for Sons of Hodir reputation are shoulder augmentations.
There are two factions with which you can align yourself in the Sholazar Basin zone. These are the opposing Frenzyheart Tribe and the Oracles. Gaining reputation with one will make you an enemy of the other. You align yourself by completing along quest chain which includes "A Hero's Burden." If you kill Zepik during this quest you become aligned with the Oracles, if you kill Jaloot you become aligned with the Frenzyheart Tribe.
"Championing" is a new way to gain reputation in Wrath of the Lich King. By wearing the tabard of a particular faction while doing level 80 or Heroic Northrend dungeons, you gain reputation for that faction. Tabards are available from the faction Quartermasters once you are friendly with the faction. The following 4 factions are those to which championing applies.
Argent Crusade - Rewards
Born from the union of the Argent Dawn and the Order of the Silver Hand, the Argent Crusade was formed to fight against the Lich King in Northrend. The primary base for the Argent Crusade is the Argent Vanguard in Icecrown, right near the border with Dragonblight. This is where you will find the Quartermaster. There are other encampments in Dragonblight and Zul'Drak.
Kirin Tor - Rewards
The Kirin Tor is a powerful sect of mages based in the city of Dalaran. The Quartermaster is in the Violet Citadel.
Knights of the Ebon Blade - Rewards
A group of renegade Death Knights, the Knights of the Ebon Blade are based in The Ebon Hold in Eastern Plaguelands. In Northrend, their main encampment is the Shadow Vault in Icecrown, which is where the Quartermaster is. Note that the Shadow Vault encampment only appears after completing certain quests in Icecrown. The also have a camp at the Ebon Watch in Zul'Drak.
Wyrmrest Accord - Rewards
The Wyrmrest Accord is an alliance of dragonflights based at the top of Wyrmrest Temple in Dragonblight. Cielstrasza is the Quartermaster.