The first promised DLC for Portal 2 has released today and includes an entirely new co-op adventure for Atlas and P-Body, and a single player and co-op challenge mode. The DLC, titled Portal 2: Peer Review, is available now for free download on the Xbox 360 and PS3 and will automatically be downloaded by Steam.

There aren’t too many details regarding this Portal 2 DLC — namely what types of puzzle mechanics Valve is exploring this time around — but we expect, if it’s anything like Portal 2, it to be tons of fun.

And not just that, Portal 2: Peer Review sees the return of GlaDOS, the masochist robot made famous by the Portal games. In what capacity she will be returning (speculation left out for fear of spoilers) is anybody’s guess, but we hope that her trademark style of humor can mesh well with the duo of Atlas and P-Body.

Here’s a brief blurb on the DLC:

In “Peer Review”, you and a friend continue the story of loyal bots P-Body and Atlas as you puzzle your way through a mysterious new co-op test track and once again match wits with GLaDOS. The free DLC also features a single player and co-op Challenge Mode, and leaderboards to compare Challenge Mode scores with friends and the Portal community.

But the free Portal 2 DLC isn’t the only good news for Portal fans. Just announced is a 50% off discount for the game, making it even cheaper for gamers to check out both Portal 2 and the DLC. The 50% off Portal 2 sale runs through Thursday on Steam, so act fast.

While it isn’t the same as giving the first Portal away for free, to be able to get not just the full main game — which includes both a compelling single player storyline and a robust co-op mode — is a deal that we recommend you don’t pass up.

The only real disappointing element of this DLC release is the lackluster single player offering. Sure, a challenge mode has been oft requested by fans of its implementation in the first Portal, but now that the possibility of a really intriguing story have been thrown into the mix, it makes simply “thinking with portals” not enough. Here’s hoping more DLC, single player DLC in fact, is on the way.

Have you had a chance to check out Portal 2: Peer Review? Would you like more co-op DLC or some support thrown singleplayer’s way?

Portal 2: Peer Review is available now, and free of charge, on the PS3, Xbox 360, and PC.