TiltFit.com, San Francisco,

TiltFit is truly a community-oriented fitness outfit. The whole idea behind its group classes is that they only occur when and where the community wants them. Guests can check out the upcoming schedule, where they might find anything from hip hop choreography to a combined vinyasa yoga and wine-tasting session to boxing taught by a professional pugilist. As long as enough people sign up for each class, or "tilt" it, then the class is on.