For the many years the NES and Famicom was in production came many accessories both licensed and unlicensed. Here I will try to cover as many as possible with reviews, scans and history.   
HARDWARE ADD-ON'S
Famicom Disk System 
The ultimate Famicom accessory with a large library of games.

CONTROLLERS AND JOYSTICKS
MAX Controller 
Updated NES controller with circular D-Pad and Turbo buttons.
Four Score Adapter 
Allows up to four players at once with a small selection of games.