. Yeah, that sure got your attention.
Before you slay the messenger, at least hear me out: Valve's returned to their pastime of amalgamating mod teams, with Alien Swarm being a UT2004 mod. Basically, it's a top-down tactical shooter, and plays kinda like Diablo, I guess. This release is, however, unique for two reasons: the source code and all of the tools that were utilized to make Alien Swarm is included with the game, including the Source SDK, promising a more user-friendly modding experience for burgeoning, would-be modders.
The second reason? It's free.