I had an unused mini-itx motherboard, left from my Perseus-2 robot project:

Robo Evolution – how to build a better robot

The specs are:
Mini ITX D201GLY2 motherboard
1.3GHz Celeron CPU
512MB RAM
IDE2CF and CF2SDCard adaptors for a 2GB SDCard based DOC (disk on chip)
M2-ATX DC-DC PC power supply 160W, so I can power the board using 12-24V DC.

The idea was to build a case and add a big 320GB harddrive. See what I managed to come up with:

The server works good, for various purposes, from file storage to permanent internet clients, and the connectivity is easily assured with a USB-WLAN card. To administrate it I use a VNC software.