VIA's Nano-ITX fits an entire PC motherboard into an impossibly small 12 by 12 centimeter format:

This board has been gestating for a while at VIA, and according to the nanoitx blog, it's evidently because of interference and heat problems presented by the extremely small form factor. But a retail board was sighted in Japan's Akiba district in late November, so it can't be too far off now.

Format Size in cm Nano-ITX 12 x 12 Mini-ITX 17 x 17 Micro-ATX 24.4 x 24.4 Flex-ATX 22.9 x 19.1 Standard ATX 30.5 x 24.4

If you've ever purchased a motherboard, chances are it was standard ATX size. The ITX specifications are essentially a VIA creation, but detailed information on the other form factors is available at formfactors.org

The Nano-ITX will be available in two versions: fanless 800 mhz for ~$300 and a 1 ghz version that requires a small fan for ~$350. You'd need a few more pieces to make a complete mini-PC, though:

• 40gb 2.5" IDE drive, $65
• 512mb SODIMM, $45
• Slim DVD drive, $45
• Nano-ITX case and power supply, ~$100 (?)

It'd be easy to build a PC smaller than a Mac Mini using a Nano-ITX board. Just don't expect a lot of power; the VIA CPUs aren't exactly barn-burners. The VIA boards are fine for basic web browsing and productivity tasks with undemanding users, but if you need more than that, you'll want a Pentium M Mini-ITX motherboard.