Wednesday October 5. That will be the day in which the Portal 2 Peer Review DLC will be released. The free mission pack will extend Atlas and Peabody's co-op adventure and add a new challenge mode for single player and co-op maps.
Back when Valve announced first DLC pack , they also mentioned leaderboards so all your friends and a bunch of strangers will all know how exactly how smart you are (or aren't), which explains the 'Peer Review' handle. That means your performance will be graded by both Glados AND the conglomerate hivemind of the The Internet. NO PRESSURE.
In more 'free stuff from Valve' news, the third volume of the Portal 2 soundtrack, Songs to Test By, is available now on the Portal 2 site , featuring such classic hits as Some Assembly Required, Your Precious Moon and Robots FTW.