A Game Boy with the following modifications:
• Overclocked to 8MHz instead of 4.19MHz (i.e. almost twice the speed, almost twice the pitch).
• Green power LED
• Internal, post-potentiometer ProSound using the headphone jack
• Removal of the speaker
More information about changing the clock speed of the Game Boy can be found on Gieskes' website, here: http://gieskes.nl/underclocking_or_overclocking_the_gameboy_classic_tutorial/
The ProSound modification was invented by Timothy Lamb (Trash80). I believe that Nanogrrl was the first person to use the ProSound modification on an old grey Game Boy, but I could be mistaken. More information about the ProSound mod can be found on the LSDJ website here: http://www.littlesounddj.com/lsd/prosound/
Here is an unmodified Game Boy playing a little riff.
Here is a ProSound modified Game Boy playing the same little riff.
Here is a ProSound, overclocked Game Boy playing the same little riff.