Initially slated for summer, Valve's first, free expansion to Portal 2 will be out for PC, Mac, PlayStation 3 and Xbox 360 next week. It's called "Peer Review" and is made for co-op.
"Peer Review" continues the game's co-op storyline, according to Valve. "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 expansion will also include single-player and co-op challenge mode that uses online leaderboards.