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.
It replaced the top of your toilet with something that looked like a sink. The water that was used to refill the tank came out of a spigot above the toilet and then ran into the sink-like thing to fill the toilet. So once you flush, use the water that refills the tank to wash your hands.