I think I've said this about 1000 times before, but I love steam. And now I love it even more thanks to Alien Swarm, an awesome FREE game. The game itself is pretty simple, it's basically a top down version of Left 4 Dead, in space. You choose from one of four classes, Medic,Tech, Officer and Special Weapons. However, not everyone can be one class, since the game requires at least one person to be tech. And you're going to want to stick together since this game, just like Left 4 Dead requires the cooperation of each player in order to enjoy to the fullest.
The game also features a persistant unlock system, much like CoD or BF. As you kill aliens you earn Xp, get enough Xp and you'll level up, level ups unlock new weapons, kits, etc. All these pieces really come together well, and really make playing Alien Swarm funand exciting. And luckily (at least in my time with the game) I never experienced any team killing or mindless behaviour.
I really can't believe that Valve put this out for free, I would have expected a price of at least $10 for such a good looking and tight playing shooter. But hey, I'm not complaining.