Valve has announced today that “Portal 2: Peer Review” will be coming to the PC, Mac, PS3, and Xbox 360 on October 4th. Owners of consoles will have to download this DLC manually, while it will be automatically downloaded for those using Steam on PC and Mac. Portal 2: Peer Review is the first free DLC release for the game and continues the story in the co-op mode. P-Body and Atlus are again woven though a maze of GLaDOS’ puzzles in this addition to the game, along with a Challenge Mode that can be played either alone or cooperatively. Challenge Mode also has a leaderboard, so players may compare their scores to friends and the the Portal 2 community at large.