Here's a great device for keeping your home energy dollars from going down the drain. A drain water heat recovery device preheats the cold water bound for your water heater by circulating it in a coil around your home's waste pipe. Warm drain water does double duty on its way out of the house by warming up incoming water before it enters the water heater.
For more on saving energy, consider:
How To: Save Water at Home
Radio: Conserve Water
The Black-and-White Benefits of Gray Water