Super Mario 64 Online allows up to 24 players join together in a single game of Super Mario 64, emulated of course. Last September, Kaze's online project was shut down by Nintendo, just days after its official launch. Its return means that whatever legal issues it ran into prior have presumably now been resolved.
The project now runs under a client called Net64 2.0. This modified emulator allows for better information transfers between players, and additional memory allocation. That means they could squeeze in some new characters, such as Sonic and Knuckles from the Sonic the Hedgehog series.
Kaze spoke about his project in an email with Kotaku, saying whether he believes, it too, will be removed by Nintendo:
I don't know if they'll go after Net64 too, but I believed that they only took down SM64 Online because it got so super popular and came out right before Odyssey anyway. I think this client will stay up for a little longer than the SM64 Online one.
Kaze has a number of other Super Mario 64 related projects going on besides this one. Super Zelda 64 is essentially Mario thrown into the world of The Legend of Zelda: Ocarina of Time. Just recently, he threw around the idea of a Super Mario 64: Battle Royale mode, as well.