Stirling engine powered by the heat of a hand.

Stirling engines are wonderful, especially the Low Temperature Differential (LTD) ones such as these.

You can make your own from a can, balloon, cardboard, some wood, string and wire, that can be powered by a candle or ice cubes.

Wonderful toys.

The mechanically inclined can use said materials to make extremely good engines, as well:

http://www.youtube.com/watch?v=W9cK_TMLvjI

I know what I want for christmas now! I have seen and made Stirling engines before, but none that are so beautiful and efficiently low-temperature differential.

Does anyone know of some other good kit vendors? Something cheaper than ~$134 would be ideal.

There's a lot more on Stirling Engines on wikipedia: http://en.wikipedia.org/wiki/Stirling_engine