KE3IJ's always excellent website has a novel receiver circuit that derives its DC power from AC mains hum and noise it picks up on its antenna. It uses a single 2N3904 in a regenerative circuit. Ideally the circuit would be better with a lower Vbe device i.e. a germanium transistor.
Taking this one stage further, I know from my experiments at VLF that the AC mains hum between a pair of grounded electrodes can be pretty high. There is certainly enough 50(or 60)Hz energy going free in the ground to power something useful. I keep wondering if I could get a few uWs of HF RF power from a low voltage crystal oscillator that would be enough to work some local stations on CW? My nearest station is 0.3km and the next nearest 3km. Now having a QSO using free power from my back garden would be rather fun.