The toilet is probably the most wasteful source of water in your home. Aqus has come up with a cool kit you can install in your existing sink and toilet to capture water run-off from the sink, clean it up, and then re-use that greywater in your toilet.
It saves a remarkable 7 gallons/person/day, saving 5000 gallons of water a year for a 2 person household. It doesn't say on their website how much the kit costs to buy/install, but you'd probably get some serious water savings out of it over time.
I love it when there are green innovations like this that just make sense and give you that "duh! why haven't we always done it like that!" feeling. This is one of them.
Learn more here:
http://www.watersavertech.com/