The Portal 2 Peer Review DLC has been released on Xbox Live Arcade.
Peer Review follows the two loveable robots Body and Atlas as they try to out-fox GLaDOS through a new co-op test track. Silly old circuit-boards.
Eurogamer's Portal 2 reviewer Oli Welsh hailed Valve's game as a "masterpiece" earlier this year.
"Portal is perfect," he said. "Portal 2 is not."
"It's something better than that. It's human: hot-blooded, silly, poignant, irreverent, base, ingenious and loving. It's never less than a pure video game, but it's often more, and it will no doubt stand as one of the best entertainments in any medium at the end of this year. It's a masterpiece."