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.

I’m not sure the Simpsons one was hidden. I watched all the commentaries a while back and if I remember correctly the one with the John Swartzwelder call was accessed just through the ordinary menu.

I’ll have to check out the Goodfellas one though.