The Super Nintendo has a distinctive and pleasant sound that I like, but only outputs a 32khz signal.
I really like projects like these using old sound chips as instruments.
[1] http://www.delek.com.ar/deflemask
[2] https://www.youtube.com/watch?v=nTDiPqkoFnc
A little over a year ago, Titan released a demo for the Genesis called Overdrive, and, well, it's fucking amazing, go watch it: https://www.youtube.com/watch?v=iQqJm14sHRY
https://www.youtube.com/watch?v=Hdlj8Vjof6c
If you're into FM synthesis in general, the Android/iPhone app Caustic has a fantastic FM synth (there's free Windows and OS X versions of it as well).
It's an interesting contrast with NEC's Supergrafx system, an obscure upgrade to their better-known Turbografx-16, which was released some months prior to the SNES and was a much greater advance in terms of raw power. It uses two separate graphics chips which simultaneously read graphics data from separate banks of RAM, which allows a lot more to be displayed. It has twice the sprites, twice the graphics RAM, and can use up to twice the resolution as the SNES. However, it's not as "well-rounded" a system as the SNES and is simpler in various ways. Unfortunately the 5 or so games that were released for it didn't push the hardware much, so you won't be able to see much evidence of its prodigious capabilities, but they're there.