Notable special features I've seen on DVDs:

* The IT crowd season 1 had leetspeak and base64 encoded subtitles (won't spoil what the encoding decodes to).

* Goodfellas has a commentary track with the central criminal and the FBI agent who arrested him.

* The Simpsons (around season 9 if I recall) has a hidden commentary track (not accessable by menus) where two of the writers fairly candidly explain the quality dip that was to follow, and have an impromptu phone call with the illusive John Swartzwelder.