Sulley has 2320413 hairs on his body.
Since Mary Gibbs, the voice of Boo, was very young and hard to keep her in place for recording, they just let her play and run around the studio, and followed her with a microphone.
There was a long time of production where Mike wasn't even in the film. They added him in to be a close companion to Sulley, a guy that he could express his feelings to.
Billy Crystal immedietlely said yes to being the voice of Mike, after regreting his decision to say no to voicing Buzz Lightyear in Toy Story.
Mike's New Car was originally supposed to be a scene in the movie.
I might make another list of facts, soon :)