Dundeejam ha created a GameBoy Audio emulator. You can download Reason Patches and load em in to Reason (if you want you can use also the free demo version) to play with a virtual Game Boy sound chip! After, download Ableton Song Files  (Reason is controlled trough Ableton to create a song).
Damn great!!!
Sample Virtual chiptune tracks: Tune #1 - which is a live performance (not a recording) of the tune built to sound like a Game Boy.  Tune #2 - which is a live performance (not a recording) of the tune built to sound like a Game Boy Plus.
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