“Eco Bath” designed by Jang Woo-seok: this seems like a very easy way to implement a water reuse system, especially in residential situations. Here’s a little quote on the subject from the Rocky Mountain Institute’s Primer on Sustainable Building:

Ideally, treated water should only be used for cooking, drinking, bathing, and cleaning clothes and dishes. Water from showers, sinks, and washing machines — usually called graywater — can be used to flush toilets or for landscaping.

"Eco Bath" sink/toilet water reuse design