I've also had a ton of fun doing this for PIC24 (the 16bit PIC family of microcontrollers), which has an incredibly straightforward instruction set. They're als

I've also had a ton of fun doing this for PIC24 (the 16bit PIC family of microcontrollers), which has an incredibly straightforward instruction set. They're also quite fast, as far as microcontrollers go and while I do love the Propeller architecture, the 8-core goodness makes them a little more complex to thoroughly understand (though their deterministic nature and the fact that you can actually count cycles reasonably easily makes them less complex than most multi-core processors)