Running linux on an 8-bit AVR: the author thinks his hand-wired board “may be the cheapest, slowest, simplest to hand assemble, lowest part count, and lowest-end Linux PC”. It is certainly a contender. Of course he accomplished it not by running Linux directly on the AVR, but by writing an ARM emulator – ludicrously slow, but it apparently does work, to the point that you can boot bash and execute commands.

I’ve occasionally fantasized about building a parallel computer using an array of STM32F103 chips – they are 32-bit ARMs running at 72 MHz. They have no memory manager but I had a similar idea for using VMs… it’s hard to see what the point would be, though, other than the experience of building something that is technically a parallel computer by hand.