If you liked Left 4 Dead..

[Ashliana]

Valve released a free Steam game with a similar premise called "Alien Swarm." It's top-down, you play with 3 other people (or bots) to make it through environments. They've adopted a somewhat "Call of Duty"-esque system where you level up, get access to new weapons/perks, and once you hit max level, can prestige several times. It's nifty. Check it out.

http://static2.videogamer.com/videogamer/images/pc/alien_swarm/screens/alien_swarm_2.jpg

[The Korean]

Fucking facehuggers on hard and insane

[StrayRogue]

You can access a FPS mode too. Really cool game with a really interesting development story behind it.

[HelloKitty]

Really fun game--I wish there were more campaigns but it's cool that a development kit was released as well. Tried hard-mode of this last night but failed halfway in the campaign due to everyone being retards and running off into diff. directions. Can also get an alien swarm hat for TF2 through achievements.

Would be fun to have a massive pvp version of this!