Valve has announced that they will be releasing Alien Swarm for free today on Steam. The game is a four player co-op top down shooter. It’s a new version of a mod for Unreal Tournament 2004, from the very same developers: Black Cat Games. Said developers are now employed by Valve and have gone on to remake Alien Swarm in the Source engine. The game comes with the complete game code and mod tools so you can muck about with it. Normal gamers can expect free achievements and Aliens, apparently.