A guy on Youtube calling himself spiritwalker47 has constructed the coolest console mod this side of Ben Heck--a relatively compact portable Nintendo 64 called the 64 Boy
. While it stretches the bounds of "handheld" like similar portable mods, it's still pretty freaking small, managing to cram the N64's guts, complete controller layout and even the RAM expansion pack into a space slightly thicker than the original gray brick Gameboy. It also has sockets for rumble and memory packs and can plug into a custom docking station, allowing for 4-person multiplayer and video/audio out. Its rechargeable battery lasts about four hours, which is pretty good considering it's running the actual hardware and powering a decently bright screen.
spiritwalker47 is currently taking offers for the thing and while I wouldn't pay upwards of $500 for it, I admit to wanting one just for the novelty and nostalgia. The DS is about as powerful as the N64 but never saw a lot of ports from the console due to different controls and architecture, so it didn't work as a portable N64 the way the GBA functioned like a portable SNES. I know a lot of GN staffers couldn't care less about the N64 but outside of my first Compaq PC and an ancient Atari 800 the N64 was my first console, so seeing this portable brings back a lot of fond memories. In any case it's a pretty amazing mod job and might even give Ben Heck a run for his money.