Like Toy Story 2, Monsters, Inc is an animated film that has been made completely on computers by a company called Pixar.
It's so good that lots of people are tipping it to win the first ever Oscar for an animated movie at next year's awards, where it will be up against Shrek.
Star Wars trailer
But the huge success at the box office isn't just because Monsters, Inc is a really good film.
One of the trailers before the film is the teaser for Star Wars Episode II: Attack of the Clones. Lots of people are going to see Monsters, Inc just so they can get their first look at the new Star Wars flick..