Toilets use fresh water, and lots of it.  Sure, you can buy a low-flow model, but you're still flushing with a (smaller) quantity of fresh, otherwise drinkable water.

Gregory Lavoie decided to get double use out of that water by hacking his toilet.  It's a neat, reversible modification: after you flush, you use the clean, incoming tank refill water for handwashing.  Your slightly soapy hand-wash run-off fills the tank, and is saved for the next flush.  

It's a simple, ingenious way to save water with little to no effort.

 

Original Idea came from www.Instructables.com