As promised, Alien Swarm has been released as a free PC game download by Valve via their Steam service. The sci-fi themed top-down co-op shooter comes with both the game as well as the source code for developers to use as an example of how to make a stand alone game using Valve's Source Engine. It contains co-op suppor for up to four players, four player classes, over 40 weapons and items and a map generating tool.

As we reported last week Alien Swarm started out several years ago as a mod for Unreal Tournament 2004 that won many prizes during Epic Games' original Make Something Unreal mod contest. The mod team, collectively known as Black Cat Games, then announced plans to release a stand alone version of Alien Swarm using the Source Engine. Valve silently brought in the Alien Swarm team members to work for their company and in between working on major Valve games the team continued to develop Alien Swarm.