Sterling engine in your PC:

http://www.extremetech.com/article2/0,1558,2272507,00.asp?kc=ETRSS02129TX1K0000532

A great idea, use the waste heat that the CPU throws off to power the cooling fan. I wonder how noisy this is? Also, sterling engines are fickle, as they use a sort of resonance between the expansion inside the cylinder and the cooling that the output fan creates. Still its a neat idea, and maybe we'll see it becoming more common, if the tech is stable (and cheap) enough.