Just stack a whole bunch of these together; they are small and energy-efficient and they do not come with Windows by default, though judging from the other posts in this thread it seems as if a 500MHz CPU and 256MB of RAM (with no possibility of upgrade) may be insufficient for your needs.
http://www.fit-pc.com/new/specifications.html
Specifications
Hardware
CPU: AMD Geode LX800 500MHz
Chipset: AMD CS5536
Display: Integrated Geode LX display controller up to 1920x1440
Memory: 256MB DDR 333MHz soldered on-board
Hard disk: 2.5” IDE 60GB
Ports:
2 x RJ45 Ethernet ports 100Mbps
2 x USB 2.0 HiSpeed 480Mbps
RJ11 RS-232 (cable supplied)
VGA DB15
Stereo line-out audio (headphones)
Stereo line-in audio / Mic
Software
General Software Embedded BIOS
Special BIOS features:
Etherboot for boot over LAN
Etherlink for BIOS update over LAN
Linux:
Ubuntu 7.10 (Gutsy) - Preloaded
Gentoo – optimized for fit-PC - available as an ISO image
Other Linux distributions are tested to work
Windows: XP Home/Pro
Mechanical and environmental
Splash and dust resistant enclosure
Material: Aluminum
Finish: Black anodized
Width: 116mm
Length: 120mm
Height: 40mm
Weight: 500g
Operating temperature: 0-70C
Power: 5V single supply
Power consumption: 3-5W
Certifications
CE, FCC Class B for Home or Office Use (see certificate)
Package contents
fit-PC computer (p/n ENC-iGLX-HDD) according to specification
AC adapter - 110/230 to 5.0 volt
AC cable (U.S. plug)
RJ11 to DB9 RS-232 serial cable
Price: $259 + S&H
S&H (first unit in a shipment): $20 inside Israel, $60 to North America and Europe, $80 elsewhere
Add half the initial S&H for additional units in a shipment.
For the record, I live in America, not Israel, and I learned about the Fit-PC via a text ad while I was looking at Gmail.
Also for all I know they might soon upgrade to Ubuntu 8.04 now that it's available, just as they recently upgraded from a 40GB to a 60GB HDD.