The Game Raccoon is a game development interoperability cartridge designed to allow you to test your own Sega Mega Drive or Sega Genesis in-development games on real system hardware.
It uses an inexpensive PIC microcontroller to transfer game images from SD card to internal memory, where the Mega Drive can read your code as if it were from a normal cartridge. It has its own internal menu system allowing you to explore the filesystem of the inserted SD card and select an image to boot. No external programmer or tools are required.
Revision 1 can boot many homebrew images from sites such as itch.io, as well as the author's own YM2612 Instrument Maker.