The first working model engine I ever had (still have, in parts, in a crate somewhere) was a compact stirling engine, roughly 10 x 5 cm at the base, fantastic gadget for a single digit year old child.

IIRC there's a New Zealand company WhisperGen(?) that use them for power from waste heat from water heaters ...

Checking now, they seem to be used as marine power generation these days, I can't find the original site from 15+ years ago.