Left 4 Dead is a PC and Xbox 360 co-op FPS where you and up to three friends control the survivors of a zombie apocalypse, fighting through different scenarios to get to an extraction point. Along the way, zombies ("Infected") attack in droves, and at pace. They're marshalled by boss-type Infected, who can do things like explode themselves, lynch you with prehensile tongues, and pounce onto you from 30 feet away. The kicker is that the AI designed by Turtle Rock, the game's main developer, varies the pace. Even if you know the levels, you're never sure when you'll be attacked.
Eurogamer: It'd be a pretty impressive way to launch it - to start with a free weekend.
Valve's Chet Faliszek: Yeah. All that's been talked about and bandied about, and I can't say if we're going to do one thing or the other yet. But definitely Steam allows us to do things on the PC side that no other developer can do, and we definitely don't ignore that. We're going to capitalise on that.