Wanna Call Pixar?
About 3 minutes into the movie, when the simulation is ended and the monster reaches for a knob on the control panel to review the videotape, just below and to the left of the knob is a little indicator which reads "510-752-3000". This just so-happens to be Pixar's phone number (creators of the film).
Monsters Created the Film
The tote board which tracks the results for each monster shows 13 monster names, most of which are the names of people who helped create the movie:
Ranft - Joe Ranft (Story artist, Additional Story Material)
Jones - Marcia Gwendolyn Jones (Story Manager)
Peterson - Bob Peterson (Story Supervisor)
Rivera - Jonas Rivera (Art Manager)
Luckey - Bud Luckey (Story Artist)
Schmidt - Andy Schmidt (Animator)
Pauley - Bob Pauley (Production Designer)
Ward - Jay Ward (Art Coordinator)
Gerson - Daniel Gerson (Screenplay)
"Luxo" Ball Spotted
The ball from "Luxo," one of Pixar's first animated shorts, appears in the film. Boo hands this ball to Sully when he's returning her to her bedroom near the end of the film.
Props to Past Filmmaker
Near the beginning, when Mike is telling Celia about his plans for her birthday, he mentions that they were going to Harry Hausen's Restaraunt. Ray Harry Hausen was a major special effects creator in the 50's. His specialty was "monsters".