As promised, Valve has published Portal 2's first chunk of downloadable content today for PC, Mac, Xbox 360 and PlayStation 3. Titled "Peer Review," the update continues the co-op story as players navigate P-Body and Atlas through a fresh set of test chambers, presumably constructed by the ever-cynical AI, GLaDOS. The DLC also contains a single player and co-op "Challenge Mode," as well as leaderboards to compare Challenge Mode scores with your buddies.

Considering how many players protested Portal 2's relatively short campaign, this update (and future DLCs) should continue to smooth things over with the community. Despite causing an initial uproar, the user score on Metacritic has improved over recent months and the game has reportedly sold over two million copies. The fresh content is readily available and it should be free of charge on all supported platforms – including the Xbox 360 and PlayStation 3.

What's more, as part of its midweek madness sale, Steam has chopped half off Portal 2's regular price. A single copy is available for $14.99, two copies are fetching $27.49 and a bundle pack of Portal 1 and 2 are going for $17.49 – in case you missed the recent giveaway. Although the original Portal will probably be free again at some point, it's definitely worth the $2.50. We haven't played the add-on content yet, but if you have, your thoughts are welcome below.