In typical Valve fashion, their next release is coming a bit later than expected. Portal 2's long-promised free DLC will be available on October 4th on PC, Mac, PS3, and Xbox 360. The "Peer Review" DLC will be automatically downloaded via Steam on PC and Mac, and will be available on Xbox Live and PlayStation Network for console players.
The add-on will add a brand new co-op test track for P-Body and Atlas, as you "once again match wits with GLaDOS." In addition to the new co-op track, the DLC also adds a single player and co-op Challenge Mode, and leaderboard support to compare Challenge Mode scores.
Along with the DLC announcement comes the third portion of the "Songs To Test By" official soundtrack, and new ringtone packs for both iOS and Android based cell phones.