Steam news announces the release of the promised new Peer Review DLC for Portal 2, adding additional co-op missions to the first-person platformer sequel for the low price of free. Speaking of low prices, this is accompanied by a "midweek madness" sale offering a 50% discount on the game. Here's word on the new DLC:

Also, FREE DLC just released! Peer Review is the first DLC for Portal 2, the year's most brain-bending action-adventure game. Featuring a devious new co-op track that continues the story of Atlas, P-Body and GLaDOS, the DLC also includes both single and multiplayer challenge modes, plus leaderboards so you can compare your portal placement and time scores with the Portal community.