From the Stirling engine description:
This is the Stirling engine that's powered by any heat source--even the palm of your hand. The engine relies on the same thermodynamic principles that powered Robert Stirling's original model in 1816, but is enhanced to operate in response to a low temperature difference between the upper and lower plates of its base. The two aluminum plates are connected by two sealed piston chambers and when the bottom plate is at least 5º F hotter than the top, the air inside the chambers expands and pushes the pistons upward. As the pistons rise they spin the flywheel, which in turn pushes the pistons back down, repeating the mechanical cycle until the heat differential--from a computer monitor or even a cup of coffee--is lost.
Here's where you can get this (not exactly, but awfully damned nice) "fuelless" Stirling engine.