A BacterialFuelCell is a device designed to allow bacteria to break down organic matter and generate out Hydrogen. One of the problems with this sort of system is that the bacteria can generally not completely break down organic matter, leaving some complex organic substances, such as acetic acid.
More recent work, however, has suggested that the bacterial digestion can be augmented by the addition of a small voltage—something like 1.2V—which enables the break down of most substances.
This kind of FuelCell seems like a good idea to me. Hydrogen is not the perfect form of Fuel—Methane would better as it has a higher energy content—but combined with a simpler AnaerobicDigestionPlant, as a final step this sort of cell could reduce the remaining slurry which can not be digested by normal means. The other obvious use for this sort of BacterialFuelCell is in a GeneratingSewageSystem.
There are also some organisms, such as Carboxydothermus hydrogenoformans, which can generate Hydrogen directly from CarbonMonoxide and Water. This again could have use quite a few uses. CarbonMonoxide will form naturally from combustion in anaerobic circumstances. It occurs naturally from various other sources (including volcano's). Of course in this form, it's really a FossilFuel, which is NonRenewable. However, in many cases natural sources will be released into the atomosphere, where they will change into CarbonDioxide anyway.