Meet the Gameboy Zero. It's a classic Gameboy shell with a Raspberry Pi's heart. But the mod isn't purely cosmetic.
Using Emulation Station, this Pi board is able to play games from NES, SNES, and the Gameboy's many iterations all up to and including Advanced. That's where things start getting tricky, since the Gameboy had only two buttons, while later systems used many more. Two of the screws in the back are tossed out to make room for makeshift shoulder triggers, while additional holes are drilled in the front for X and Y buttons which were salvaged from an SNES controller.
The coolest part by far is the custom cartridge. Some resoldering and a little work with a Dremel put a fully functioning SD to Micro SD adaptor inside one of Nintendo's square grey shells, allowing the Pi board to read it.
Additionally, the mod adds USB charging, a significantly better screen with full colour, a much improved rechargeable battery, and a mini HDMI output in case you want to play these games on a larger screen.
How does all that stuff fit inside an old Gameboy shell? Those first gen Gameboys were almost criminally underpowered, not to mention that most of the space inside the shell was taken up by four AA batteries. The smaller lithium cell helps a lot but it's still a tight fit.