If you can solder and have a basic understanding of electronics, you can add MIDI output to your DS, DS Lite, Game Boy Advance, or GBA SP. The Midify chip also interfaces with other electronics -- apparently anything that has some kind of user input. For just $35, this chip adds a MIDI port to the DS.
It doesn't turn the DS into a MIDI sequencer or allow you to output MIDI, rather, it adds a MIDI in port to the DS, allowing you to control the DS via keyboard. Confused? Imagine the DS is the microwave seen here -- it has been fitted with the Midify chip.