Alien Swarm is being given away on Steam. A remake of the original and from the makers of L4D I believe.

 

It's a top down, action, 4 player co-op game.

 

I've played for several hours and it is pretty addicting. You'll die a few times (lots if you're on hard, even more on insane) but it just makes you want to figure out what builds to use and what strategies to have to finish each level.

 

http://store.steampowered.com/app/630/

Great lan-game, not so much online as people aren't able to cover their area and just mill around. Guess you can skype it though.

lame!  I have a mac!