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Some sort of FM synthesizer for Gameboy Color. Sounds quite interesting!
http://www.geocities.jp/submarine600/html/fm.html
Website's in Japanese though, but just click on the 'gb_fm.zip (2009.04.10版)' link above the picture of the interface to download the rom.
Have fun!
PS: I didn't make this!
Yeah, eventhough I don't have a clue how to use this thing yet! But I studied Japanese at the uni for a couple of years, so if there's enough interest I might translate the instructions or something. 
Translation:
Move the cursor with the D pad. Currently selected values are reversed/white.
Press B to trigger the note. Press A to trigger the note raised one semitone.
Press L/R to change the octave.
Press select to turn "repeat" on/off, which produces a continuous note.
Press start to edit a waveform. Press start again to exit.
You can the W and M values in the formula. T is the time variable (read position in the waveform) and goes from 0-31.
The amplitude goes from -7 - 8. The output waveform is the carrier modulated by the modulator waveform.
The picture below shows the same parameters as above but with higher resolution.
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My own notes: M is the modulator multiplier (overtone frequency) and W is the modulation depth.