I am getting a lot of hits looking for an Intel Atom Server. Here are a couple of alternatives using ARM or Power PC chip. I found this company in Toby Velte's Green IT book.

Here is some information on Excito's Bubba Server.

Bubba® – The Mini Server

Comfort and control!
All you want from a full-size PC you get from our mini-sized Bubba server, designed to fit your home better than an always running PC. With Bubbaserver you get a product in a small form factor but still fully featured:

Torrent downloader: Turn off your computer and relax! Let Bubbadownload your large files instead of leaving your PC on all the time. Bubba also handles HTTP and FTP downloads.
File server: Allows you to access all your files from work or school at all times. Time to live life smarter and forget about USB memories and other portable media.
Easyfind service: Access your BUBBA with <choose a name>.bubbaserver.com, from anywhere in the world. (BUBBA|TWO only)
Streaming media server: Put your MP3 collection in the 'music' folder on Bubba, and all your music will be instantly available to all ITunes compatible (DAAP), or UPnP compatible players. Squeezebox support coming soon.
Router: Use Bubba as a router and firewall for maximum security (BUBBA|TWO only).
Web, e-mail and FTP server: Fully featured server functionality allows you to turn off your PC.

All this and more for less than 13 watts.