Viatech.com makes them. They are based on small form factor mobos of about 7x7", and fit on the standard ATX/mATX mounting hole pattern so they can fit just about any type of case. Very low power - generally don't need any (or very little) active cooling. Great for HTPC and such. Performance wise, they don't give a Socket A Duron of over 1 GHz any competition except in the noise dept.
. The new Tt Mozart TX case can hold one of those Plus a regular full-sized ATX mobo so they can split the load...

People build them into all sorts of things - I've seen them built into toasters, humidors, lunch buckets, purses, brief cases, various plasic models and RC toys, etc. etc...
.bh.