These are similar in concept to the
Gumstix machines. Gumstix has two lines of gum-package sized computers, the Waysmall machines at 84x37x15mm and the Netstix machines at 103x35x25mm. There are five in all.

If you want to roll your own, you can choose from 9 motherboards, each 80x20mm and featuring your choice of attachments, SD cards, USB, bluetooth, a cooice of 200Mhz, 400 Mhz and 600 Mhz Intel XScale processors. Each motherboard comes pre-flashed with full Linux v.2.6. They start at US$129.
There are started packs containing power supplies, cables, and expansion boards. You can add WiFi boards, audio boards, ethernet and more.

I am hoping for one in my Christmas stocking.