Lots of people have built human powered generators. I like this one for two reasons:

1. It's designed to harvest waste energy. (It helps your body stop your knee from swinging at the end of the forward swing, something your body uses energy to do anyways.)

2. The evaluation included serious medical studies to determine the amount of metabolic energy it "takes" from the human.  (to get a watt of power, it increases the human's load by less than a watt. By comparison, a hand crank generator takes 6.4 watts from the human for each watt harvested)