The first set of free Portal 2 DLC, titled Peer Review, launches on PC, PlayStation 3 and Xbox 360 on 4th October, developer Valve has announced.

The pack continues the story of Aperture bots P-Body and Atlas as they make their way through a new co-op test track, once again trying to outwit GLaDOS.

On top of that, there's a new single player and co-op Challenge Mode, and leaderboards are now live too.

While you're waiting, the third and final installment of the Portal 2 soundtrack is now up for download free of charge, including Jonathan Coulton's ace end credits tune, Want You Gone.