Want a sequel to Mario Paint? This disk drive add-on is your answer.
The Nintendo 64 did not have an overabundance of proprietary ports on the console. The one that most gamers are familiar with, the memory expansion port, would go on to be a portal for developers to improve resolution, textures, and other technical elements. However, the bottom of the console housed a port labeled EXT, which went nearly unused by Nintendo for the duration of the console's life cycle. In Japan, however, this port was utilized by none other than Nintendo's infamous Nintendo 64 Disk Drive.
The Nintendo 64 Disk Drive, or 64DD for short, was conceived during a time when cartridge sizes were thought to be at their technical limits. Though other console manufacturers had transitioned to CD media, the fallout over the SNES CD led Nintendo to develop another magnetic media disk drive. After years of delays, the 64DD was finally made available to the Japanese public on December 1, 1999. While Nintendo did offer the drive in limited quantities at retail, the system was sold mainly through a subscription service. Originally, the subscription service would have gamers paying off the 64DD and other services at 2,500 to 3,000 yen per month. This was later changed to to a yearly plan priced at 30,000 to 39,600 yen per year. The starter kit included the 64DD unit, modem, modular cable (for connecting to the internet), 4 MB Expansion Pak (which was required to use the 64DD), and member disk. 64DD software was also included and delivered to subscribers on a monthly/bimonthly basis.

Many of the features that Nintendo's ill-fated add on touted may not seem like much by today's standards, however the possibilities that the machine had were far beyond what most believed would be possible at the time. Once attached to the N64, the system will automatically check whether there is a cartridge in the console or a disk in the drive. The disks themselves were akin to Zip disks (which were more or less extremely beefed up floppy disks) and held 64 MB of data. Unlike the CD-ROM based PlayStation and Sega Saturn, these disks also offered up to 38 MB of writable space, which gave developers enough room for in-game changes, game saves, and more. The 64DD unit also contained a 36 megabit chip with integrated fonts and audio data files, giving programmers less stress as they wouldn't have to worry about cramming all of this information onto the already limited N64 cartridge.
The 64DD also had network capabilities through a service called Randnet. Players could compete with others online (complete with stats, ranks, and win-loss records), observe other players' game sessions, play pre-release games, send messages, surf the web, listen to music, view sports info and results, check weather and news, utilize a print mail service (after creating something in Mario Artist, for example), and even receive up to five email addresses. The online features were robust and provided unique content for 64DD owners.

One of the main benefits of the 64DD platform was that it could potentially lower costs in a similar manner to the Famicom Disk Drive. With the Disk Drive in the picture, developers and publishers had a few different options when it came to creating games for the N64: cartridge only, disk and cart combination, or disk only. With the disk and cart combo, if developers, for example, were making an American football game, they could release the main version of the game on cartridge. After one year goes by, they could release a disk containing updated rosters, player data, and even slightly updated visuals. If games were released solely on disks, this would be an even cheaper way to output games.
Despite the lower costs all around for 64DD development, the system was poorly received by the development community as well as gamers in Japan as a whole. Even though some developers may have been interested in developing for the platform, Nintendo did not actively push for more 64DD development. When it was all said and done, roughly 15,000 units were sold and a total of nine games were released for the system, many of them published and developed by Nintendo and/or their affiliates.
Here are all the games that were released for the system:

Kyojin no Doshin 1(Doshin the Giant)
Mario Artist: Paint Studio
Mario Artist: Talent Studio
Mario Artist: Communication Kit
Mario Artist: Polygon Studio
Sim City 64
F-Zero X Expansion Kit
Nippon Pro Golf Tour (Japan Pro Golf Tour)
Kyojin no Doshin: Kaihou Sensen Chibikko Chikko Daishugou (Doshin the Giant sequel)
The Randnet service was active for less than two years (from December 1999 to February 2001) before being discontinued.
The potential of the 64DD was wasted by the inability to get the product out of the door and reluctance on Nintendo's part to draw in developers and publishers. Many former 64DD projects, including the likes of Mother 3, Fire Emblem 64, Dobustu Banchou (Animal Leader, aka Cubivore on GCN), Ogre Battle, Paper Mario, Zelda Gaiden (later became Majora's Mask), and Ura Zelda (Master Quest) would be either scrapped, moved to N64 cartridge, or ported/recreated for another platform entirely. As time progressed, the capacity of the Nintendo 64 cartridges grew and the need for the device waned as each year passed. By 1999, developers and publishers were getting more out of the N64 than they had originally thought was possible
Many of the concepts behind the 64DD harken back to the Famicom Modem and even the Satellaview for the Super Famicom. The EXT port was not used officially by Nintendo outside of Japan (although pirated machines in Hong Kong for the N64 made use of it) and the Nintendo 64 Disk Drive would be Nintendo's last aggressive move into the world of online connectivity. Although the system was a commercial failure, many of the features would go on to be included in the gaming machines that we use today. Its legacy is not limited to hardware, but also its experimental software. In particular, the Mario Artist series served as a basis for many modern Nintendo features and game such as the Mii system and WarioWare series.