http://www.pcgamer.com/super-mario-64-online-turns-the-classic-into-a-co-op-platformer/

Mario 64 wasn't designed to be played cooperatively, but then, it wasn't designed to be played on PC, either. But thanks to the magic of emulation it's easy enough to boot up, and thanks to the magic of modding you can now play the game cooperatively – though whether you'd want to is another question.

In the mod – which you can download here, though you'd best watch the tutorial first – you can play as pretty much any character in the extended Mario universe. Better still, they all have their own movesets, so Princess Peach can hover for extended periods, and Waluigi can traverse steep slopes.