What about Solar Water Heaters?

Solar water heaters -- also called solar domestic hot water systems -- can be a cost-effective way to generate hot water for your home. They can be used in any climate, and the fuel they use -- sunshine -- is free.  There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don't.

http://energy.gov/energysaver/solar-water-heaters