Joey Nariano [animal-style] demonstrates his GBC Gameboy Foot Controller.

The Original Pixelh8 Music Tech Game Boy Synthesizer is a cartridge that fits into an unmodified game boy, letting you control the Game Boy’s on-board sound. You can edit the wave duty, pitch sweep and volume envelope as well as new additions such as unison, 5th power chords and octaves.
This updated version V2.0 allows for [...]

What happens when you combine age-old contrapuntul style with Gameboys?
One possibility is Joey Mariano’s Gameboy Madrigals:
Early composers of the madrigal used only two to three voices to create counterpoint. Realizing the opportunity for reinterpretation through the gameboy sound chip, Joey Mariano set out to create a body of chiptune madrigals inspired by early RPG games, [...]

This video demo’s Midify, a DIY project that lets you add a MIDI port to a Game Boy.
The Game Boy SP is running a synthesizer cartridge that would normally have to be played on the Game Boy buttons. With Midify installed it can be played with a MIDI keyboard, sequencer or whatever.

Littlescale demos syncing Nanoloop, a real-time Gameboy sequencer/synth, to Ableton Live:
Part 1

Nanoloop on a Gameboy (whose audio is coming in on track 2) is synchronised to the drums playing in Ableton (track 1). This is to test out a new interface I have been working on that syncs Nanoloop to a MIDI clock.
Part 2

Nanoloop on a Gameboy (whose audio is coming in on track 2) is synchronized to the drums playing in Ableton Live (track 1).
Says littlescale - “This is to test out a new interface I have been working on that syncs Nanoloop to a MIDI clock.”

You’ve probably seen GameBoys used to create 8-Bit music, but have you seen a GameBoy Drum Machine?
kBANG ver 01 uses the new GameBrain cartridge, using SMD technology with 1 megabyte Flash and 16 out 8 in ports.

Promised with the ext release: speed control, save patch and mute channel - this was done in a week [...]