I've got a spare mini-itx system I built a couple of months ago that is just collecting dust and that's just not right. The system is the smallest form factor PC I've seen. This thing is smaller than the MAC mini. It uses the MAPOWER S3000 case you can see the specs on it
here.
Here a pic next to my small Ericsson T series phone and a set of std headphones.

You can see some additional pics
here.
Here's a quick blast of how it is configured.
C3 1Ghz
512MB PC2700
60GB slim latop drive
USB 2.0
Firewire
Dual boot with Windows XP and Linux.
This lil machine plays all the games in KAWAKS at full speed. (ROMS not includedP
I'm asking $400 shipped within the continental US. This isnt a bad deal considering the cheapest I could find the case and mobo for was $300 today.
Feel free to ask any questions.
As always thanks for looking
-Goz