Two years ago, Valve hired a group of modders behind the popular Unreal Tournament 2004 co-op mod called Alien Swarm. The team has since contributed to popular games such as the Left 4 Dead Series and Portal 2, but on the side, they were hard at work on a Source version of Alien Swarm, and the fruits of their labor will be released on Monday - for free.

In addition to providing the game free of charge, Valve will release its complete code base, including updates to the Source engine SDK and full Steamworks integration to provide "more insight and examples" for aspiring modders and small-time developers.

Alien Swarm is described as a tactical top-down shooter that "thrusts players into an epic bug hunt featuring a unique blend of co-op play and squad-level tactics." There's also mention of over 40 weapons and equipment with "countless" loadout configurations, four unique classes, eight characters, and 64 achievements.