Anyone else playing Alien Swarm? It's a four-player cooperative top-down shooter game from Valve, where you fight your way through swarms of aliens to complete mission objectives. There's eight characters to play as, divided into four classes (Officers who give a passive damage boost, Special Weapons with the big guns, Medics who heal, and Techs who hack consoles and doors), and there's something like forty unlockable weapons.
It's really fun. And the best part? It's free! 100% free. Not as a sale or promotion or whatever, just permanently and totally free on Steam.