Greywater recycling

Current designs of toilets use fresh water for flushing.  Sink Positive is a retrofit sink that can be placed on top of any toilet.  It allows greywater to be used to flush the toilet, maximizing water usage.

This design has been implemented in Japanese homes, as can be seen below:

The mechanism is simple and requires no additional pumps or mechanical materials.  As the diagram shows below shows, the fresh water used to refill the reservoir simply passes through the faucet first before being added to the tank.  The runoff after washing is then used to refill the bowl.

Advantages:
1. Large amounts of water savings
2. Space-saving; a separate sink is not necessary.
3. Toilet leaks are easy to spot. Whereas for conventional toilets, water refilling is done internally, here, any noticeable sink dripping means a leak.

Disadvantages:
1. For the uneducated, it looks like you're using the dirty water from the bowl to wash your hands. 
2. The toilet bowl impedes access to the sink, requiring the user to stoop to use it.
3. The flush button needs to be pressed every time anyone wants his or her hand washed.

The last two issues are easily fixed with a different design, but it will remove the elegance of a marketable retrofit toilet cover.

Source: Sink Positive