I'm a big fan of Valve games and Steam.  I'm also a big fan of zombie games, and Left 4 Dead is sure to be a new one coming (though technically none of the "zombies" are zombies, but people infected with a form of the Rabies virus).  It's going to be a co-op game where you and 3 other people finish objectives against a swarm of "zombies" where an AI director spawns randomly on the map every time (so you never know what's coming).  You don't have to be the survivors all the time, you can also be one of four of the zombie bosses.  If you want to hear more about it go to ...

http://www.l4d.com/

or

the fan-site (which is very informative, I would rather check this one out than the first) http://www.left4dead411.com/