My former roommate had the idea a few months ago to build a ‘green’ PC, which would be very low power, easy for anyone to use for things such as email and office applications, and (of course, the clincher) cheap. So he went ahead and did it… it worked out quite well. Its a shiny lil box, actually. The performance isn’t that bad either, and it is in fact power efficient.
Its running Ubuntu on it (of course, its supposed to be cheap!), and he mentioned some plans to research patches and tweaks to make the Linux kernel more power efficient, and include other peoples power-related patches and such in there as well.
Anyways, visit his site, it doesn’t have a whole lot of content there yet but he was planning on adding some more…