After the failure of the SATA and USB ports on my Intel D945GCL Atom board, I decided to build out a new file server. Sticking to the Atom theme, I decided to go small and get the CompuLab FIT-PC2. This little toy uses the Z530 1.6Ghz CPU that apparently uses only 6 watts of power. I'm assuming that means *without* a hard drive installed.
The PC sits on top of the enclosure on my bookshelf taking up 8.5" x 5.0" x 6.5" amount of space. This is not only power efficient, but space efficient since I am using 4 x 1TB drives. 4TB total (theoretical), ~2.6TB in a ZFS raidz. If I were to have purchased the 2TB drives, it would be even better.
Doug's blog on the FIT-PC2 gives a good overview on the features of the device and what works. There is no wifi driver and Xorg doesn't work, so you may want to install OpenSolaris on another machine before installing the internal HDD. My server is headless and uses the built-in gigabit ethernet, so I don't care about those issues.
Links and prices
CompuLab FIT-PC2 Diskless Nettop PC
Mediasonic (HF2-SU2S2) Pro Box 4 Bay Enclosure
Samsung 1TB 7200 RPM
4 x $85 = $340