06/25/2007:
Of recent interest to me are the small form factor ITX motherboards. Most have a full set of video, audio, PCI, PATA, SATA, single RAM slot (DDR2 usually). The smallest, and as of this typing, not available yet, is a VIA PICO-ITX board 10cm x 7.2cm (4in x 3in) motherboard. Also available are tiny power supplies the size of an ATX connector. Mini-Box is a nice resource for purchasing and the VIA link is a nice resource of general information.
I first started investigating these little buggers when I was considering designs for a cluster a friend of mine is planning on building. Looking at price vs speed, a generic PC will usually beat the speed at around the same cost by double if consider single core and quadruple if you consider most of the systems are dual core. The benefit comes in at the size. I envision an octet of these guys with their rear connector breakouts all coming out the side of a PC case. I'd pick one of those fancy new cases that has a plastic side panel that I could easily machine and get a look at all the nice little mobos. Loading each machine with two 80GB sata (in raid 0, for speed), 1GB ram and then put a nice KVM and Gigabit switch in the box and you'd have a 12GHz 8 core cluster for under $2,500 in the space of a standard PC tower. Ideally two of these and you'd have 16 dedicated systems for $5,000.
Alternatively, one of these boards and a nice 4/8 sata hardware raid PCI card and you'd have a nice home brew Network Attached Storage (NAS) or large file server. I'd run raid 5 with as many 500GB drives my budget/raid sata could support. Ah, but those are fantasies, I have far too many projects going at once too start another one.