The name, Mike Wazowski, was came up with by the Muppet Man himself, Frank Oz.
Boo, who was originally named Mary, was originally going to be six years old, as a smart-alec kid who's older brothers play tricks on her a lot. They changed her to be younger, so that she'd be more dependant and more loving to Sulley.
In one of the many script ideas for the movie, Sulley was a janitor who is too afraid of humans to be a scarer, Randall is the top scarer, and Mike is his assistant, but still Sulley's friend.
Due to the ENORMOUS amount of hairs on Sulley's body, it took a total of TWELVE HOURS just to make ONE frame with him in it.
If you look closely at Boo's drawings, she wrote her real name on it, "Mary".
This was the last Pixar film to have bloopers in the end credits. One of them even includes Rex :D