Looks like the heckuva popular new NES cartridge + software + hardware hack called the Chip Maestro is off to a fantastic start as it has already gained backing from over 50 people who want to buy the thing, resulting in over 3000 dollars already paid toward the maker's way.
So how's it work? Check the link above for a demonstration video, but the gist of it is using computer software or a standalone piece of MIDI hardware (keyboard, etc.) to control the NES and directly create your own chiptunes without emulators. Sounds pretty good too, whether they are free-styling it or using some pre-made sheet music/MIDI tracks like the music from the Zelda or Mega Man series.