Wireless RF Energy

Its been known since the 1900s that electrical energy can be extracted from the Radio Frequencies in the air. RF energy causes tiny amounts of electrical induction in every metal it touches, and by using some diodes and capacitors, it can be extracted and stored; free electricity from the air!

Note that unless you are standing next to power lines or a powerful radio transmitter, the amount of energy you can extract is very tiny, so not very useful.