These low-cost 8051 microcontrollers with USB look interesting.

This guy has many elegant AVR projects.

This is a classic. Dmitry wrote an ARM emulator for the 8-bit AVR ATmega microcontroller, and successfully built what is probably still the slowest, cheapest, single board Linux computer.

He's also written an ARM Thumb emulator for the ATTiny85:
http://dmitry.gr/?r=05.Projects&proj=08.%20uM0