Developers of the Power-Pipe Drain Water Heat Recovery (DWHR) system claim it can reduce residential water heating costs by 20 to 35%. The DWHR is basically a drain pipe with a water jacket built in. To install it, you cut away a section of ordinary drain pipe and insert the DWHR version. The water in its water jacket comes from the cold water pipe. The output of the jacket goes to the cold-water input to your hot water heater. Essentially, the hot water going down your drain then preheats the cold water going into the hot water heater so there isn't as much work left for the hot water heater to do.
DWHR systems are said to work with any type of water heater and can be used in homes, multi-residential buildings and commercial applications. Canada has been offering DWHR systems for sale online for over six years. The developer also says a typical unit will reduce greenhouse gas emissions by about 400 lbs/person/year when displacing natural gas water heating.
The technology is distributed by RenewAbility Energy USA and is sold in large, big-name home do-it-yourself retailers.
The Power-Pipe system can be viewed here:
Consumer video: http://www.youtube.com/watch?v=N3F3k0zeglk
Installation video : http://www.youtube.com/watch?v=-jE0C7bsTv8