Raphael Assénat created his own reprogrammable cartridge for the Sega Genesis so that he could give a try at writing his own homebrew games. I've seen his webpage before, in particular, his experimetns with webcams. Good stuff.
Hardware-wise, the Genesis is kind of a cute computer: It's got an 8Mhz 68000 processor, 64KB of dedicated video ram, and a separate Z-80 chip to control the sound processor. A veritable super computer compared to the machine I've been playing with.