Tomorrow, Valve is releasing Alien Swarm, a top-down co-op alien shooter originally developed as an Unreal Tournament 2004 mod, absolutely free on Steam. They’re also releasing the Source SDK for the game, essentially making the source from the project free for other game developers and modders to use as an example to work from.

Why? Because they’re Valve, that’s why. And judging from this trailer, this is going to be an awesome game. We’re going to have to play this at the next WTC Game Night.