As I reported in Saturday's news roundup, Alien Swarm has been released on the Steam network for the bargain price of absolutely nothing. Nada. Zilch. In fact, this actually qualifies as an essential pickup and gesture of extreme goodwill rather than a normal deal! Go on. Download it.
Back in the day, Alien Swarm started life as a popular Unreal 2004 mod that allowed players to team up and slaughter aliens from an isometric perspective. In fact, it was so successful (and well-coded) that Valve decided to acquire Black Cat Games and set them to work on the likes of Left 4 Dead and Portal 2... but allowed the team to work on their original vision as a fully-fledged project.
Alien Swarm provides four distinct character classes, eight characters, persistent experience levels and items and "countless" loadout configurations... as well as the titular swarms of ravenous beasties. You'll need solid teamwork, good spatial awareness and more than a little luck to triumph against the hordes of bugs that Alien Swarm throws at you... but 64 Steam achievements and over 40 weapons will make it well worth your while. It's an impressive blend of arcade shooting with RPG-style squad mechanics that'll keep many gamers away from the big-name titles for some time.
Not only is the full game available for free, but a fully-featured set of mod tools are also included in the package that essentially allow the community to prolong the gameplay experience indefinitely. Expect new maps, new weapons and entire new campaigns to materialise in the fullness of time.
Put in the simplest of terms: this is a thing that you want.