I'm fascinated by them. From the very moment I figured out how they work, I have insisted that they should be used more widely for all kinds of power solutions. I believe NASA have used them in space. I wonder if they have tried nano-engineering a bunch of Stirling engines onto a silicon wafer.
Stirling engines are beautiful things. I've got one and I run it from time to time. They certainly spin very nicely from a small burner or even the heat of a computer monitor. It is a real pity that they are not going to 'save the world' as they are not as efficient as internal combustion engines, in terms of $/watt and watts/Kg of engine and watts/Kg of fuel burnt. Also, they don't have much in common with the Carnot cycle even though descriptions of Stirling engines usually start off with a description of the Carnot cycle. The Diesel cycle is more efficient, though not nearly so mechanically beautiful.
For anyone curious how they work, get one and have a play. And then get out a physics book and study the thermodynamics as it is an absolutely fascinating subject.
One day I am going to try to build a Carnot engine - mainly because the physics books say it can't be done. It would need complex microcontroller control which very neatly makes this thread *picaxe related*!
It's a steam engine the attached however is a stirling engine as I also have a bit of a thing about them. It seems several car manufacturers including Volvo and Ford have invested much mony in research but as yet no production models.
To make this a Picaxe thread - before Dippy thrown it out  I have an idea for a boat engine on the stirling principle but using water as the piston so there are no seal issues and driving the displacer with a motor controlled by picaxe. the intention there is the picaxe provided speed control of the main motor and the stirling part drives a sort of water ram jet to power the boat.