I have a watch that's wound by the movement of my arm during the day. This does not violate conservation of energy. Movement is energy. It uses a complicated mechanism to extract a little power from the movement it experiences, and uses this power to charge the watch's battery.
I'm thinking of scaling this up. You would wear a bracelet on each wrist, and an anklet on each leg. And as you walked, ran, and went about your day, they would each trickle charge a small battery. You might only get an AA battery's charge out of each one bracelet and anklet, but that's an AA battery's charge more than you had before. You could power a small gadget with just your own movement. Maybe a cell phone, an embedded computer, or a PDA? The "free" power would be nice, anyway.
The energy would come from making it sliiiiiightly harder to move, but not in such a way that would be a big deal. And frankly, we could use the exercise, nationally.