A simple, ingenious grey water system called Aqus just hit our radar this week. It makes so much sense - it collects greywater from the sink to reuse for flushing.
You and your guests use both the sink and toilet as usual, but internally, the system is at work, recycling away...a two person household can reduce their water usage by 5,000 gallons a year by using the Aqus.
Here are a few pluses:
And a minus:
We don't have vanity sink and we don't think the Aqus would look too hot sitting next to the pedestal style one we do have.
If you are willing to spare some cabinet space to save mucho gallons of water the Aqus might work for you - it's available from Vivavi.
More info at the manufacturers site: WaterSaver Technologies