Just read about this knee-joint powered generator. The story is that this device is built around a sports knee brace. It uses the "wasted" energy of your lower leg swinging as you walk. The claimed output of 5W is enough to fully charge a 2300 mah NiMh battery in half an hour of walking. It's not ready for production yet, but it looks much more promising than any of the other techniques for generating power from the human body.