This is so freaking cool. A quote is in order:
This has to be the most jaw-dropping NES mod ever … putting a whole system-on-a-chip NES console clone into an original game cartridge. Talk about how to demonstrate the advances in shrinking electronics over the years! Modder Kotomi took an original Super Mario Brothers cartridge, gutted it and somehow squeezed in all the circuitry, chips, power switches, twin joystick ports and composite video and stereo audio sockets. And it even looks pretty neat. If I was wearing a hat, I’d tip it in Kotomi’s direction.