I've wondered if there could be a way to use the Earth's heat by drilling a small hole in your back yard a few km deep and somehow embedding a tiny generator down there to take advantage of the heat differential between various layers of rock. Maybe a Stirling engine? I'm thinking like a long thin tube filled with liquid that boils at low temps. The liquid would boil, expand, rise up through an insulated tube, condense at a cooler layer and circulate back down. A little generator could use the constant motion to power a house.