Tiny Linux Thin Client
The ICOP eBox III is an ideal Linux thin client with either a 200 or 800mhz CPU it's about the size of 2 DVD cases stacked ontop of each other. The 3800 is available for only $238 with an 800mhz CPU and 256MB RAM, it can boot from LAN via PXE or you can add an EmbedDisk to the 44pin IDE.
The 3853 is approximately 3 DVDs thick, has dual 10/100 RJ45 LAN plugs, and has space for an internal 2.5" HDD.
The 2600 and 2620 are both 200mhz and are 3 and 2 DVDs thick, with 2 RS232 adapters and 2 RJ45 LAN adapters respectively.
More information available here:
http://www.wdlsystems.com/ebox/ebox.shtml