The Cataclysm expansion added eight new reputation factions, four that are shared between the Horde and Alliance, and two more for each. The purpose of this guide is outline the methods of gaining reputation with each faction, as well as describe the faction rewards available by gaining reputation with them.
The four factions that both Alliance and Horde members can gain reputation with are:
The Earthen Ring, a Shaman-based organization dealing with the aftermath of the Shattering and attempting to defeat the plans of the Twilight Cult and their master Deathwing.
Guardians of Hyjal, a Druid-based organization defending Mount Hyjal and the world tree Nordrassil from assault by the Twilight Cult and the elemental lord Ragnaros.
Therazane, the Earth Elementals who reside in Deepholm and aren't very happy to see the players until they demonstrate their usefulness.
Ramkahen, denizens of Uldum who are descended from the Tol'Vir and still share their antecedents' mission to protect the Titans' secrets.
In the Twilight Highlands, there are two factions:
The Alliance struggles to unite The Wildhammer Clan, Wildhammer dwarves dealing with internal dissent, the depredations of the Dragonmaw orcs, and making sure they have enough ale to drink.
The Horde seek to free The Dragonmaw Clan from the control of Mor'ghor and his fel orcs while making sure the Wildhammers don't get too rambunctious.
In Tol Barad, the Alliance aids the Baradin's Wardens in defending the prison island from the Horde-assisted assault team of Hellscream's Reach.
Very Short Summary of Faction Rewards (details available in their respective faction discussion)
Guardians of Hyjal, The Earthen Ring, Ramkahen, and Wildhammer/Dragonmaw clans provide lvl 83 i333 blues at Honored, lvl 85 i346 blues at Revered, and lvl 85 i359 purples at Exalted. Each of these factions also provide a Helm enchant at Revered.
Therazane provide Shoulder enchants, Lesser ones at Honored and Greater ones at Exalted. At Revered, the Therazane have several i346 rings available for purchase.
Baradin's Wardens and Hellscream's Reach provide trinkets and weapons available to purchase with Tol Barad Commendation badges, which are awarded with daily quest completion. There is an i333 trinket available for 40 badges at Honored, i346 weapons available for 85 badges at Revered, and i359 trinkets available for 125 badges.
And for those looking for something besides slot gear, Ramkahen has camel mounts available at Exalted, while Baradin's Wardens and Hellscream's Reach provide an assortment of mounts, pets, battle standards, and other battle-appropriate gear.
As in Wrath of the Lich King, players can wear tabards for the respective factions in order to gain reputation in Cataclysm Level 85 Dungeons, which includes Lost City of Tolvir, the Halls of Origination, and Grim Batol in addition to all Heroics. The tabards are available at Friendly reputation, and can be purchased from the faction quartermaster. The exception is the Baradin's Wardens/Hellscream's Reach tabards, which are simply ports to Tol Barad base camps and will not earn reputation when worn.
In the descriptions below, reputation gained from dailies and questing may be noted. The numbers listed are before any racial or guild rank bonuses; having those bonuses will accelerate your reputation gain accordingly.