Valve releases Alien Swarm free via Steam. Alien Swarm was originally created as a mod for Unreal Tournament 2004 by Black Cat. "In addition to game, Valve will also release the complete code base for Alien Swarm. This includes updates to the Source engine SDK and full Steamworks integration. If you've ever thought about developing a mod on the Source engine with Steamworks, this release provides more insight and examples for using Steamworks in game production."
The game is a co-op, top-down shooter with one primary focus, blowing up aliens. In the game you can play as one of eight different characters in one of four different classes using up to 40 different types of weapons and equipment.