The Human Powered Workshop featured a number of projects whose fuel source is you. I am a huge fan of these machines and applications --- sometimes you can get something Rube Goldberg would have been impressed with and sometimes you an very elegant approaches.
When I think human-powered an image of someone cycling away comes to mind. These folks have taken things many steps further:
A Street Cleaner: the Dirt Anhilator
A Milk Warmer---(of course)
In 2005, the MIT based project, Bicilavadora won the MIT IDEAS competition with their cycle-powered machine to replace the old stone-and-river method used in many places around the world.
More at WorldChanging
and if you're into building this stuff Instructables has some great projects as well as Howtopedia, Appropedia, and Maya pedal.