Motherboard From $200 Everex Wal-Mart PC
Its on sale, in a dev kit, sort of, it comes with the motherboard, as you see it in the picture, and a copy of the gOS Linux distribution that was meant to be run on the motherboard. The best part, it's a mere $60.
That $60 gets you a motherboard with a 1.5GHz VIA C7 CPU, and it's a mini-ITX motherboard, which means you can stuff it in any number of places or items. The RAM and the hard drive storage are up to you, but I would recommend a large CF card for storage.
So, you can spend the full $200 and get the full system with a case, HDD, RAM, DVD burner, keyboard, mouse, and speakers or you can spend just $60 and get the mobo with integrated processor and a copy of the OS. Personally, I don't have a bunch of extra PC components just lying around, so I'd buy the whole system.
Low-cost board runs Linux, Google apps [via makezine]