Today I'll be borrowing an item from Josh Spear, who borrowed said item from Apartment Therapy.
People normally think of grey water collections systems in large-scale proportions, concentrating mainly on landscape irrigation in industrial or mixed use settings. This, however, is an ingenious little invention from Vivavi: The AQUS Toilet System.
The AQUS system collects used water from your sink and collects it to use as flush-water for your toilet. According to the website, the system can reduce water usage in a 2-person home by 5,000 gallons per year. Josh Spear also brought up a good point that the system is non-invasive enough to be easily installed in a rental. Much easier than asking your landlord for solar voltaics on your roof!