Valve has announced it will release a Source Engine version of the Unreal Tournament 2004 mod Alien Storm free of charge via Steam later today.

The game comes from the same same people who worked on Left 4 Dead and now Portal 2, and thrusts players into an epic bug hunt featuring a unique blend of co-op play and squad-level tactics.

With your friends, form a squad of four distinct IAF Marine classes. Plan your attack using an unlockable arsenal of weapons with countless loadout configurations against a wide variety of aliens.

In addition to the 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 should be available as a free download later today.