Super Mario 64 Plus is a modified version of the PC port of Super Mario 64, which takes the original and adds loads of stuff like 60FPS, bug fixes, a ton of in-game options and, best of all, “The ability to continue the level after getting a star.”
Don’t think of it as a completely different Mario 64, then. Just a kinder, gentler version of the original, only for a more civilized age. And while the new features are welcome, it’s the extensive options that are the most interesting, as they let you mess with pretty much everything within the game itself, from Mario’s jumping abilities to camera sensitivity to graphics settings.
There’s more information on the project here.