Well, here's a nifty mod. A guy by the name of Hailrazer decided he wanted to play Nintendo 64 games on the go. As no portable N64 exists, he built one himself.
He used the body of a Game Boy Advance, stuck in a Gamecube joystick and a couple of camcorder batteries and cut down the N64 board to fit. What resulted is a surprisingly professional-looking device that he dubbed the N64Boy Advance. I love it.