HotHamWater
Member
Initially slated for summer, Valve's first, free expansion to Portal 2 will be out for PC, Mac, PlayStation 3 and Xbox 360 for free. 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.
Kotaku
Also, Songs to Test By vol. 3 came out today.
Edit: Official PR.
FREE PORTAL 2 DLC SHIPPING OCTOBER 4th
"Portal 2: Peer Review" Available on PC, Mac, PS3 and Xbox 360
September 29, 2011 -- Valve, creators of best-selling game franchises (such as Half-Life and Counter-Strike) and leading technologies (such as Steam and Source), today announced the free DLC update for its bestselling game franchise Portal 2 will be rolled out across all regions on Steam, Xbox LIVE and PSN starting October 4th. The free DLC, titled "Portal 2: Peer Review", 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.
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.
To hold you over until the release next week, visit http://thinkwithportals.com/ for the final installment of the Portal 2 soundtrack Music to Test By.
Portal 2 is a hilariously mind-bending adventure that challenges you to use wits over weaponry in a funhouse of diabolical science. For more information on Steam, please visit http://www.steampowered.com.