A while ago, my wife wrote about how we are trying about finding ways to save money on energy. The Power-Pipe is a way to recover the heat energy from waste water. To install this, you would replace a section of the main PVC drainage pipe, with a copper pipe, that has smaller pipes coiled around the large one. Cold water is run through the small pipes, where it will acquire heat from the drain pipe, and then it would run into the hot water heater.
Supposedly the Power-Pipe will increase the cold water from 10 to 24 degrees C. This is possible since 90% of all hot water energy goes down the drain, and the Power-Pipe can recycle as much as 60% of this energy.
Unfortunately, the website for the US residential distributer is not up. On Canadian site, the Power-Pipe costs between $270 - $1200.