The Nintendo emulators available for Android are pretty sweet, I used to love playing the old Pokemon games. The only problem is, most of these games are made for use with a gamepad, not a touchscreen. So when playing, fingers would get in the way of what you're trying to see.
Chad Boughton loved playing emulators so much, he took apart his old Gameboy to use as a gamepad for his Galaxy Nexus. He took the bluetooth interface from a Wii remote and stuck it into his Gameboy, wiring everything up. It essentially becomes a Gameboy and Wii remote in-one. By using the already available Wii Controller IME app which is on the Play Store, he connects to the Gameboy via Bluetooth. He also cut out the old Gameboy screen and fitted a dock which will comfortably hold his Galaxy Nexus.
The Gameboy doesn't just work with Nintendo emulators though, you can of course play other games. It's a pretty sweet mod which actually looks bloody awesome.
A commenter also mentioned sticking an NFC sticker to the gamepad which has preset settings. So when he docks his Galaxy Nexus into the Gameboy, it connects automatically. What a brilliant idea.