Red Dead Redemption.
The "Outlaws to the End" Co-Op Mission Pack will feature six multiplayer missions and be compatible with 2-4 players. The free download will also add Trophies and Achievements for players to unlock. (For details on some of the missions, click past the break.)
In addition to the free co-op mode, Rockstar Games plans on offering two additional downloadable content updates "over the summer." It's unlikely that these offerings will be free, however, as the press release carefully notes "pricing" will be announced in the future.