Really small, but maybe a bit limited:
http://en.wikipedia.org/wiki/SheevaPlug
http://plugcomputer.org/
I host multiple sites on a SheevaPlug and USB based harddrive. It works fairly well, except for a few problems with occasionnally needing to drag laptop with a USB cable to get a debug terminal in case it stops working.
That said, it was the best small server I've used. I had an Efika for a while (while I was waiting for MOS for it) and a first gen fitPC. Both systems worked fine, but the extra ram found in the SheevaPlug works more.
That said the newer fitPCs have better specs:
http://www.fit-pc.com/web/ .
In both cases the SheevaPlug and FitPCs would need you to setup and install the webservers, email servers (including spam filters), and even the OS. There are some decent packages like ehcp.
