Easter egg: On the main menu of every disc it is possible to press up and select certain objects on the picture, and activating these causes an animation involving them. Also if you leave the menu idle for a while animations will happen using the characters or events of the episodes featured on the disc.
Easter egg: On Disc 4 on the Complete Fifth Season, select "Sweet Seymour Skinner's Badasssss Song", and up in the top left corner should be the words '100th episode'. Highlight and select them. You'll get a showing of the episode's animatic with a secret commentary by Matt Groening and most of the show's animators about the production of the episode. They also talk about the effect of the California earthquake on the show's production.