On the Rain-Slick Precipice of Darkness: Episode 1
http://www.playgreenhouse.com/
supposed to be a solid 8 hours of gameplay
edit: you can also download the game on xbox live, but since it doesn't have crazy system requirements i'm downloading it on pc.
double edit: i've personally enjoyed the game from what i've played so far, but if you're not a fan of penny arcade i doubt it would be much fun. there is a demo version though so you can try it before dropping the 20 bucks
motherfucking triple edit: i already beat the game, probably took about 6 hours haha (fuck you, it's summer and i had no plans today). the game is entirely linear so there's no replay value. but the 6 or so hours that you do get are a pretty good time. i'd probably give it a 7/10
also, don't buy the game on xbox 360 if you don't have an HDTV. i've read that it's very difficult to play on a standard definition TV, and there is a lot of text in this game.