You might also consider a Seagate Dockstar. It can run Debian and can boot from a usb flash drive. They are no longer made but can be found on ebay for about $40 US. It's similar to a Sheevaplug but only has 128 MB RAM. I've had one running a webserver since last September. See here:
http://jeff.doozan.com/debianhttp://forum.doozan.com/