Sleep is Death is a two player game where one person creates a story and the other person plays it. Everything he does has a reaction and new parts of the game are introduced on the fly.
Check out the main website to learn how it is all done.
I picked up this game soon after release and I think its great. I'm currently finished making one story involving the mafia and about 95% finished with a different story. The creation tools are excellent and easy even in a tight spot. I'm finishing my music now.
If anybody here wants to play either of stories tell me and we'll give it a try. Its a very unique game.