Super Mario 128 was a technology demonstration shown off by Nintendo representatives at the Spaceworld trade show in 2000. This demonstration was made to showcase the processing power of the Nintendo GameCube, and featured 128 Marios running around a circular plane, picking up boxes, knocking one another over, and jumping around. In addition, the tech demo showed dynamic permutations of the terrain, showing how the Marios could react to sudden bumps forming beneath them and pool together when the plane resembled a giant bowl, while a meter at the bottom of the screen displayed the approximate workload of the system hardware at all times.
Although it was primarily a tech demo, announcements made by Shigeru Miyamoto over the years surrounding its display led to a lot of confusion over whether or not Super Mario 128 was also a video game under development. The reason for this is that "Super Mario 128" was used as a working title during the late 1990s and early 2000s to reference possible sequels to Super Mario 64. Although a direct sequel to Super Mario 64 was in development during the lifespan of its console, the Nintendo 64, that game always had the working title of "Super Mario 64 2", and was actually intended to utilize more or less the same game engine as its predecessor in a similar fashion to Super Mario Galaxy 2. Super Mario 128 was supposedly going to be an "innovation" of the Mario formula, so different from what people had seen before that it couldn't be shown at E3 lest someone steal the idea.
Instead, the game languished in development hell for over a decade, and was even moved from GameCube to Wii, before Miyamoto finally admitted that it was never going to be released in a recognizable form. Although another Mario title, Super Mario Sunshine was released during the time that Nintendo was apparently working on Super Mario 128, Nintendo representatives have confirmed that it is an entirely different game.
However, the development of Super Mario 128 was not for nothing, as bits and pieces of the game's engine have contributed to other Nintendo games released over the years: Miyamoto has said that the most major was Pikmin, in which you control many semi-autonomous characters at once; a physics engine from the project was used in Metroid Prime; and the ability to walk on spherical surfaces was used in The Legend of Zelda: Twilight Princess and Super Mario Galaxy.