MAKE Magazine pointed to an informative piece on how to create your own reprogrammable cartridge for the Sega Genesis/Megadrive. It can be used for many things, such as running demos, homebrew games and music on the real console.
Sounds like a cool project for me to take on when I get caught up on stuff. My Genesis is the first console I ever bought, so I’ve never gotten rid of it. This would definately bring some new life to it for me 🙂