Left 4 Dead 2's amusement park campaign on the most recent episode of GTTV. If you skip past the first half of the episode found after the break (which, inconveniently enough isn't available as a standalone video), you'll get a few minutes of new gameplay from the upcoming zombie-murdalizin' sequel.
As you might imagine, there are zombies. There are also amusement park rides. As if clowns weren't terrifying enough! Come on, Valve!