LSDJ MIDI Boxes
I used to wonder if this would ever be created, and years later, here it is. These boxes let you hook up your Game Boy (loaded with Little Sound DJ) to a MIDI keyboard. In English, that means you can control your Game Boy’s sounds using a certain type of music keyboard, rather than tediously programming songs into the software. Granted, the cryptic nature of LSDJ is part of the novelty, but being able to actually control the Game Boy’s audio in real time - rather than using emulation-based plug-ins for your digital audio software - is awesome.
The boxes come in two flavors - one to control a single Game Boy and another to control up to four. Since these are so awesome, you’re going to have to pay up, though - $151 for the former and $255 for the latter, to be specific. Worth it.