In an excellent article over on the Register, Damon Hart-Davis writes about how he massively cut his electricity bill for his home server set-up. You can read the original article here:

How I built a zero energy cost, zero carbon home server

The main item that stood out for me in the article was the usage of SheevaPlugs which I had not heard of before. They are basically a network-enabled mini computer in a form factor that looks like a power transformer you'd find on a laptop charger. After a bit of searching online I found a tutorial on how to set up Fedora on one:

How to install Fedora on a SheevaPlug and boot off a SD card

It looks like a SheevaPlug would make a very nice low-power replacement for my test server at home (which is currently an old PC tower).

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Updated 2022 : note that the above post was originally published in 2010, but is left here for archival purposes.