Nintendo is happy enough with its own Super Mario Maker, but modder extraordinaire Kaze Emanuar figured, hey, we could probably do with a Super Mario 64 version as well. So he made one.
Super Mario 64 Maker is a download for the PC that you can play via an emulator (such as Project 64). You download it, fire it up, and just... get creating. You place objects in the world using the same camera viewpoints you would to play the game, and you can drop pretty much anything in there, from enemies to checkpoints to various different pieces of terrain and backgrounds (and you can change the size of a lot of those things as well).
It obviously isn't as fast or as neat as Super Mario Maker, but then, that was a professional product made by Nintendo. This is a custom thing made by a fan, and in that context of what he had to work with, it's amazing.
To download it (or to view/upload levels), check out the descriptions in this video.