Stirling engine.

Behold!

It is a moving sculpture project built to raise eco awareness and stuff like that. (Well, it could be. In fact it was made out of boredom)

That's a really working low temperature difference Stirling engine built almost entirely from packaging trash - things that are manufactured just to be thrown away. No high tech trash (such as old hard drive parts). It is powered by cup of hot water underneath (the final version will be powered by the heat from lamp that is highlighting it).
Surprisingly, it spins at up to 120 RPM on fresh cup.
Low temperature difference Stirling engine has undeserved reputation of requiring expensive things such as nice bearings, graphite piston, and stuff like that.

How Stirling engine works: There is a plate in chamber. When the plate is high, air is in contact with bottom lid, which is hot. Air expands, pushing the membrane, turning the shaft, and moving the plate down. When plate is low, air is in contact with top lid, which is colder. Air contracts, pulling the membrane, and turning the shaft. See more info at http://en.wikipedia.org/wiki/Stirling_engine

Materials:

Top and bottom plates: can lids.
Piston: I used rubber membrane from broken gas heater. On second thought, should have used polyethylene from food packaging.
Chamber wall: cut from some plastic can.
Displacer: cut from styrofoam tray.

Rods, axis, and other mechanics: paperclips, 8 paperclips total.
Rubber tube: I only needed it for leak testing.
Spacers: pieces of wire insulation.
Low friction washers: cut from thin transparent plastic cover on some packaging.
"airtight" holes for displacer rod: just regular holes. With some liquid soap for sealant (it is sticky enough to hold enough pressure for it to be totally airtight). It doesn't matter - the leak area is very small, and leaks here, if any, are totally insignificant.
Glue: hot glue, applied using soldering iron.

'Flywheel' is temporary just to balance it and make sure it works. I replaced it with plain piece of thick copper wire.

Tools: needlenose pliers, wire cutter pliers, knife, pin, caliper, soldering iron (used only for applying hotglue), compass (the one that you draw circles with).

Keywords: Stirling engine, Carnot cycle.