Ubisoft have released their Animus Project 1.0 DLC update for Assassin's Creed: Brotherhood's multiplayer, which includes a new game mode - Advanced Alliance - and the new map Mont Saint-Michel. The update is available for free right now, with Animus Project Update 2.0 planned for release in January. That update will also be free, and will also include a new map and a new multiplayer mode.
The update is available for Xbox 360 and Playstation 3.