I stumbled on a site ages ago that I apparently forgot to include on this blog, but I recently found a couple of sites related to it.
Graywater Heat Recovery and REI both have products that recapture the heat from warm water that goes down the drain, transferring that heat to water coming out of the faucet. All told, this technology allows a homeowner to significantly reduce the amount of energy it takes to heat water.
The original site I found is EcoDrain.
Somewhat related: I'd love to have temperature-controlled faucet lights. Why? Funsies!