The FamiCoin is an accessory for the Nintendo Family Computer (also known as Famicom in Japan) that was manufactured by Nintendo and released in Japan. The FamiCoin was placed over the D-Pad on the Famicom controller and helped provide extra grip and possibly helped to prevent Repetitive Strain Injuries associated with gameplay (sometimes referred to as Nintendo Thumb).
The FamiCoin came packaged with two rubber pads that came in various colors. The pads could then be placed over the D-Pad on the Famicom controller. Additionally the package included two stickers that could be placed on top of the rubber pad. The stickers came with different designs on them, most commonly Space Invaders.