a very small Arduino running at 16MHz using the 8-bit CPU ATmega32U4.
A cheap Arduino Mega clone from Seeedstudio, running a 16MHz 8-bit ATmega 1280 CPU.
a very cheap Atmel AVR chip, using an internal 8MHz crystal, flashed with an Arduino compatible bootloader.
Cubieboard (Raspberry on steroids)
not a microcontroller in the common sense, more a complete micro PC including SATA and 96 GPIO Pins, running on an ARM Cortex A8 CPU with 1GHz.
Leaflabs ITeadMaple (a Maple clone)
running on a 72MHz ARM Cortex M3 CPU