DVD Talk has a DVD Easter Eggs Database. I’ll have to try some of these with the DVDs I own.
Such as how to view the “Marcus Cole: Dead or Frozen?” segment on the Babylon 5 Season 5 set.
Or how to see an animated 11 minute segment about the Polymorph on Red Dwarf.
And I’d always heard that there were blooper reels hidden on the B5 sets, but I couldn’t find them. Here’s how to navigate to them on the Season 2 DVD set and the Season 3 DVD set. (There must be bloopers for the other years; I wonder where they are.)