A few years ago I started playing around making my own sterling engines. A lot of fun. Basically there's a lot of excess radient heat everywhere... your hand, the exhaust from your computer, running Bryce on a dual core using 100% CPU.... The cement along roads, the roof of a house.... Lots of things give off heat which can be converted easily using a sterling engine. Even the heat of your hand can power one.

http://en.wikipedia.org/wiki/Stirling_engine  A link about them.
http://www.stirlingengine.com/ A company that makes them.

You can buy kits if you want to play around with them, or build them yourself. There's a type of competition among sterling engine enthusiasts to make a sterling engine out of anything.. even tin cans, straws, string, and a heat source....

If you want to make these engines work for you in terms of saving engery in a practical sense, you'll need a lot of them. So they are not practical to run your house on them, not yet anyway. But they are getting smaller and more efficient every year. Current talk is like having them small enough to fit under every shingle of a house's roof, converting heat into energy from every shingle. We're not there yet technology-wise yet, but that's the goal among some enthusiasts.