First off, we'll go with the slightly ominous, but also slightly cool:
Biological fuel cell - A device with a lot of useful potential for implanted devices, such as heart monitors or blood sugar detectors (for people with diabetes). Not so bad, though a little odd if you start thinking about 10 gallon drums filled with blood to power heavy machinery.
Then you get into the nightmare fuel:
Carniverous Clock - A clock that powers itself by capturing insects, dumping them in a vat of bacterial solvents and harvesting their life force to power its terrible difference engine.
The matrix wasn't THAT far off, it looks like. Except in the future the robots wont bother keeping us alive, they'll just be breeding us like cattle and tossing us into big dissolution vats after sucking out our blood for their fuel cells.
I'm going to go cry in a corner now.