From Sustainablog:

Researchers have found a way to generate electricity from bacteria in a cow's rumen.

Researchers at Ohio State University found that a pint of a cow's bacteria-infested rumen juice produced about 600 millivolts of electricity--about half the voltage of a rechargeable AA-size battery. How many cow's guts it would take to run a good sized city is an open question.

The microbes generate electricity in a microbial fuel cells by passing electrons to an anode, the negative electrode of a fuel cell. The electrons flow from the anode through a wire, producing a current, to a cathode, the positive electrode of a fuel cell, where they combine with hydrogen ions and oxygen to form water.