If you're one of the three or so people in the world who need a quad-core on a mini-ITX board, Jetway's JNC62K is just the right thing for you.
The diminutive 17x17cm board is based on Nvidia's 8200 chipset and it supports AMD low power CPUs, such as the nowhere-to-be-seen Phenom 91x0e and Athlon 48x0 series parts with a TDP up to 65W. There's one DDR2 slot and the board supports up to 2GB of memory, six USBs, VGA and DVI out with an HDMI dongle, four SATA ports and one IDE connector.
There's no PCIe slots, just an ancient plain PCI, so don't count on upgrading graphics, ever. The 8200 will have to suffice, but it should do just fine for a sexy and slim HTPC. The board itself is a bit more expensive than a similar mATX board, but if you're into the whole mini-ITX case modding scene, you'll probably be willing to cash out the 88 quid Jetway wants for it.
You can check out the bit-tech review here.