Plain and simple, I think the Sink Positive may be one of the best green inventions I've ever used. After you flush any toilet its water tank refills for the next flush. All the Sink Positive does is route that refill water first through a spigot (so you can wash your hands with it -- it's clean, just like it comes from the sink!) and then it sends the now "gray water" back into the toilet tank. I just can't believe how intuitive, simple, and water-saving it is.
Actually, most impressive/scary is how much water actually goes into each toilet flush (even a low-flow!). Whenever I've used this, I always have at least twice as much possible wash time as I need. Totally amazing and every toilet should have one. :) I'd give it 10 stars if I could!