I've been working on my third book, meaning I've been playing the hell out of my Genesis, which, along with my other systems, is set up in the family game room. If someone wants to watch TV in the game room while I'm playing the Genesis, I simply plug in the headphones and keep plugging away at whatever game(s) I'm playing. This got me to thinking that the headphone jack on the Genesis is a highly underrated feature.
I also think the Super Game Boy on the SNES is an underrated feature (peripheral in this case), since it gives the games color, brightness, and, obviously, a much bigger playfield (making it much easier to see and play the games).
What do you guys (and gals) think are some other underrated console features (and peripherals)?