SF0 ("SFZero") is self-described as a "Collaborative Production Game. Players build characters by completing tasks for their groups and increasing their Score. The goals of play include meeting new people, exploring the city, and participating in non-consumer leisure activities." It's a sort of alternative reality game, and is based mostly in SF — but increasingly people outside of the city are playing the game.
SF0 was created by Playtime (Ian Kizu-Blair, Sam Lavigne, and Sean Mahan), a non-profit organization. The project began in the summer of 2005.