Very nice: C Data Solutions, in the UK, has produced a compact flash computer -- a Motorola ColdFire-based computer in a CompactFlash card.
They also make a couple of interface boards (a backplane and a bus controller module to provide power and translate some signals) and an RS232/BDM header that lets you talk to the system from your PC.
It's fairly pricey, at US$200 for the PC module, but it seems they're aiming this at the consulting and prototyping market. Their $600 development system (PC module plus backplane, bus controller, BDM header) would be a great start to many consulting projects.
That said, you could go a long way with a PC104 system -- there's a bit more competition in that market so you should be able to get a cheaper deal. Some of them are small too: here's a little 40MHz 186 system for $129, for example.