EcoDrain heat exchanger
File this one directly in the "why didn't I think of that?" folder. As the bathroom gets more and more eco-friendly, the EcoDrain is stepping in to take advantage of all the hot water that goes to waste each time you shower. Put simply, this user-installed heat exchanger transfers heat from hot shower waste water to cold incoming water, essentially cutting water heater usage by as much as 40 percent. Of course, we've never seen a plumbing job that was anything close to simple, but for savings like this, it may just be worth the trouble.
Source: engadget.comAdded: 2 April 2009