Wow, here's a Friday surprise. Valve just announced that it's releasing a full version of Alien Swarm, a co-operative shooter based on an old Unreal Tournament 2004 mod, next week. And it's free.
The cooperative, third-person alien shooting game will be available next Monday. According to the Steam product page, Valve hired a members from the modding community (not unlike what it did with Portal) and set them to work on making the game in the Source engine. The mod was as hard as a drill-sergeant's hug, so expect a nasty, life-draining bug hunt through a ravaged colony, leveling your grunt with your friends. Alien Swarm will also support most of Valve's middleware: Steam Cloud, matchmaking, statistics-tracking and 64 Steam achievements.
It's encouraging to see Valve leaping into a genre other than first-person. Alien Swarm grows Valve's reputation for being an aggressive talent scout within the industry. We learned in October that Valve is probably working on a Defense of the Ancients-style game after Icefrog, one of the primary minds behind the mod, was snatched up by Valve and later reported that he was " leading a team " for the studio.
All the details are here: http://store.steampowered.com/app/630/ . If you can't wait until Monday, there's a heap of cheap, similarly-genred games already on Steam: Alien Breed: Impact , Alien Shooter , Zombie Shooter .