There was a great post on Lifehacker about using water collected from a rain barrel to flush your toilet, rather than the clean drinking water that is normally used. The comment thread is really good too.
So what about this: wouldn’t it be cool to have a house with two sets of water pipes, one for clean, one for grey water? Or maybe just a house that saves all its grey water to use later. Perhaps in a tank near the top of the house, pumped up there by the solar panels or wind turbines on the roof? And the grey water could be reused for a lot of things, and sould have to be overflowed into the sewer system sometimes. Or it could be used to water the lawn or something. Sprinklers shooting off of the roof? I think that would be cool.