I have a Magnic Light in the rear. It works off the eddy current created by the alum rim and the magnets inside the light itself, which then turn and create usable current. I have it mounted on an extension off my rack since the light needs to be along the rim. Since the rim would get in the way somewhat if mounted on the brakes, this puts it where it is completely visible. No wires, no batteries. While the claim is "free energy", nothing is free. While I would challenge anybody to say they could feel the difference, that energy is coming from somewhere. I like to think it is coming from otherwise wasted energy.

And how do I tell it is working? I just look for the reflection in a parked car beside me.