The new Teensy is ridiculous, especially for the price.
I built a portable NES emulator around one. It has a 3.2" SPI LCD, a port for a real NES controller, and a speaker for the audio. It works great, and is not even maxing out the CPU! It's using somewhere around 70-80% of it's capability.
For those that don't know, emulating the graphics of a NES is very CPU-intensive. Running the same code on a Mega2560 got me about 1 FPS. It might have even been less than that, I can't remember.