Valve has announced a new game called Alien Swarm and it is completely free.

Alien Swarm originally started out as a Mod for Unreal Tournament 2004 and eventually landed the team behind it jobs at Valve. They have now turned the total conversion into a full release game and they are offering it for free on Steam right now. The team has worked on porting the project over to the Source engine in between working on the Left 4 Dead series and Portal 2. In addition to the cool new game, Valve has released the SDK so would-be developers can get their hands on the same tools used to create the game.

Alien Swarm is a top-down shooter that features four player co-op play. You can choose from 4 unique classes with 8 unique characters that can use over 40 different weapons and equipment. You can check out a trailer for the game on its official site. Alien Swarm is available now for the PC absolutely free on Steam.