It sounds like someone's taking, the... well... urine on this one, but scientists in the UK claim that they've managed to convert urine into workable fuel cells. Don't start storing your urine in jars, though. Well, if you must, go right ahead, but you'll be waiting for a while to see any power results out of your own waste water, as the current results only offer a small amount of energy.
That's the beauty of a urine powered fuel cell though; there's an awful lot of excreted water that's otherwise just waste water, so there's both plenty of potential and a lot of material to test with. The BBC reports on the research, entitled "Urine utilisation by microbial fuel cells; energy fuel for the future" by Ioannis Ieropoulos, John Greenman and Chris Melhuish, quoting Dr Ieropoulos as saying that he was
excited by the potential of the work. Through this study... we were able to show that by miniaturisation and multiplication of the number of MFCs into a stack and regulating the flow of urine, it may be possible to look at scales of use that have the potential to produce useful levels of power, for example in a domestic or small village setting."