IMHO One of the coolest if not particularily useful accessories Nintendo made for the NES system is R.O.B. - Robotic Operating Buddy - this little guy stands about 9" tall, and "watches" TV - the robot is controlled by special flashing sequences in the video games. Sensors in his "eyes" detect the video signals and direct him to perform certain tasks.
R.O.B. doesn't like modern LCD TV's, apparently he relies on the refresh rate of older CRT televisions to detect the signals.
My R.O.B. is complete except for his claws which attach to his "hands" and help him hold the Gyromite tops - if you have a set of these claws, please contact me.
R.O.B. had a fairly short life, and in fact only two games were released which made use of him. The "Gyromite" game shown came with R.O.B. in the NES "Deluxe Set" (see below) and had R.O.B. help you press buttons on the second controller by moving spinning gyro-tops between posts on the Gyromite accessory.
The other game "Stack Up" is much less common because it did not come with R.O.B. In this game, R.O.B. stacks up colored blocks.
R.O.B. was included in this "Deluxe Set" version of the NE. This included the control deck, two controllers, a light gun, R.O.B. and the gyromite accessories, as well as the Gyromite cartridge and the Super-Mario/Duck combo cartridge.