Family Computer 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. It's unknown if Nintendo ever had intentions on making an NES Disk System.

Floppy disks had the advantages of being cheaper. Besides that, the disks were rewritable, making saving easier.

Games
Here is a list of Mario games released for the Disk System. Those in bold were released only for the Disk System. Those not in bold were also released on cartridge format for the Famicom.
 * All Night Nippon Super Mario Bros.
 * Donkey Kong
 * Donkey Kong Jr.
 * Famicom Grand Prix: F-1 Race
 * Famicom Grand Prix II: 3D Hot Rally‎
 * Golf
 * Golf: Japan Course
 * Golf: Japan Course (Gold)
 * Golf: US Course
 * Golf: US Course (Gold)
 * I Am a Teacher: Super Mario Sweater
 * Return of Mario Bros.
 * Super Mario Bros.
 * Super Mario Bros.: The Lost Levels
 * Wrecking Crew

Trivia

 * Mario and Luigi also appear in the Famicom Disk System BIOS. The Famicom Disk System BIOS also make a cameo appearance in Paper Mario: The Thousand-Year Door, on Grodus' computer.