I find this fascinating, and hope they overcome the technical issues that currently are associated with it.
http://en.wikipedia.org/wiki/Stirling_engine
Low Temperature Stirling Engine running on solar power, heat from hand etc.
http://www.stirlingengine.co.uk/ks90-solar-low-temperature-stirling-engine-42-p.asp
Yup, read that a while ago when researching water injection. That lead me to this; http://en.wikipedia.org/wiki/Six_stroke_engine which in turn lead me to the stirling engine. All great stuff but you can lose track of time very easily on wiki!
I was briefly told about them in a Thermodynamics course, they're neat little things. Is it only me who laughed at the first picture on the wiki site thinking "Thats very Austin Powers"
Cool. If you want a small one, buy this MSI board :)
http://global.msi.com.tw/index.php?func=newsdesc&news_no=591