Ok, for those who doesn't know Alien Swarm, it's a free game on Steam:
http://store.steampowered.com/app/630/It's pretty much Source [F**** Valve] and you could classify it as a 3rdPerson L4D but, with aliens and a little customization.
The game's actually fun and harder than L4D, represents a cool challenge, plus, the achievements are awesome, some of 'em are kinda impossible lol.
Try it if you haven't, what could you possibly loose? Nothing, it's free