This video shows a mechanism that appears to do a simerlar thing (I can't detemine from the video entirly how it works) to a powerball.

http://www.youtube.com/watch?v=a49d5cJOGQ0&feature=channel_page

 

When using my powerball it occured to me how efficent it was at converting human motion into rotation and I thought that it would be intresting to find out at what rate electrisity could be generated with one. The world record is to get one rotating at 16.5khz in speed.

 

In the comments of the above video (in with all the crank comments) I saw a sugestion a bike could be made with this mechanism.

The rotation will get a lot harder once you put a load on it. How much you get out will depend on what you're using to drive it. You'll get somewhat less electrical energy out than mechanical energy in, of course.