Famicom Disk System
The Famicom Disk System is an accessory for the Famicom. It allowed the Famicom to play certain games on a floppy disk rather than a cartridge. Floppy disks had the advantages of being cheaper. Besides that, the disks were rewritable, making saving easier.
Sharp Corporation, a Japanese Electronics and Domestic Appliance company, created the Twin Famicom, a Famicom combined with a Famicom Disk System into one piece of hardware, but it was only released in Japan. It's unknown if Nintendo ever had plans to make an NES Disk System.
Here is a list of Mario games released only for the Disk System: