In “Peer Review”, you and a friend will continue the story of loyal bots P-Body and Atlas as you puzzle your way through a mysterious new co-op test track and once again match wits with GLaDOS. The DLC also features a single player and co-op Challenge Mode, and leaderboards to compare Challenge Mode scores with friends and the Portal community.
From the Portal 2 blog:
An all-new co-op adventure starring Atlas, P-Body and GLaDOS, the free DLC “Peer Review” takes place an unknown period of time after the events of Portal 2. You and your co-op science partner are once again awoken and assembled for a dangerous new mission. Did we say “dangerous”? We meant routine. Everything’s fine. That slight note of terror you’re hearing in GLaDOS’s voice is probably just an audio calibration error. Anyway, on with your suicide mission. And when we say “suicide mission” we mean it would be suicide not to take this mission. Because it’s so safe. Everything’s fine.
“Peer Review” is being rolled out across all regions on Steam, Xbox LIVE and PSN as we speak. The free DLC is available to all owners of Portal 2 for download on PS3 and Xbox 360, and will be automatically downloaded via Steam on PC and Mac.
And if you’re not an owner of Portal 2, don’t sweat it, we’ve made it easier to become one: To celebrate the release of the free DLC, Portal 2 is on sale for 50% off on Steam through Thursday.