On Disc 3, from the Main Menu, press 8 4 7 6 3 on your remote. This will reveal a menu chock full of audio outtakes!
On Disc 4, from the Main Menu, access the 'Special Features' option. Once on the Special Features Menu press 7 4 2 on your remote. Each time you do, a different sketch will be revealed!
While viewing the following episodes press the 'audio' button on your remote to reveal a 5th audio track containing a second commentary not accessable through the menus:
Disc1 - "Stark Raving Dad"
Disc2 - "Lisa's Pony"
Disc3 - "Bart the Lover"
Disc4 - "Seperate Vocations"
On Disc 2, access the episode "Treehouse of Horror II" and select subpicture 5 in the subtitle menu. When you reach the 10 minute mark and the 12 min : 44 second mark there should be a pencil that you can click on. Click it to reveal some hidden pencil sketches!
All easter eggs were generously submitted by Satelk