Everyone else check out this incredible game from Valve. It's not one of those "Pretty good for a free game" deals, it's a fucking sweet game that just happens to be free. Hey Robert Kodick, you feel that? That's Valve's foot kicking you squarely in the junk!
The gameplay is super fun and frantic, with lots of equipment and weapons to unlock. I haven't even gotten a chance to play co-op yet, but I have a feeling that's where the game truly shines. But don't take my word for it, watch this video!
As a side note, if the game crashes for you when starting up just update your video card drivers to the latest. That fixed all my troubles.