This toilet system is so environmentally ingenious that I don't know why it hasn't been standard for years: shower, sink and bath water is collected and pumped into the in-wall tank of the toilet to be reused for flushing. Even if no flushing occurs, the water tank automatically empties itself every twenty four hours to prevent bacterial growth, with the extra precautionary measure of a constant bombardment of low-powered
ultrasonic ultraviolet radiation.
Thumbs up for offering a simple way to cut down on water usage with negligible personal impact. Double thumbs up for not reversing the concept.
Grey water toilet system by Alison Norcott [Design Awards via Freshome]
Update: The page says "ultrasonic" but that's likely a typo, as many commenters have suggested.