You know those TV Games things? (The pseudo-consoles built into controllers)? The Radica (as we call it) is a blue pseudo-Sega controller with a Genesis on a Chip inside it. It's apparently simple enough to hack it to take a cartridge. It uses something like 60 ma/h by itself, and it already runs off of batteries and outputs composite video.
Its not actually in the controller (although the controller is definitely massive enough to house the tiny board), theres a little mini-genesis-deal thing that had the main board and some stupid board with an LED and powerswitch.