DVD Region 1, Starburst Edition 3.3:

There's a hidden commentary for Dog with Two Bones by David Kemper and Ben Browder! To find it, do the following:

From Anya:  With a DVD player & TV, you can find it by pressing the audio button on your remote.  (With my [stlscape's] setup, I have to start playing the episode as I normally do, and as soon as the episode goes full screen, press the audio button on my remote.)

From IriS:  If you're watching on a computer, go to the Special Features menu and hover over the right side of the screen, across from Deleted Scenes.  Two bones will appear:  that's the link to the hidden commentary.

Thanks to stlscape for the tip!

DVD Region 1 Vol. 8: Durka Returns:

Additional footage is accessible through the film itself. Simply watch the episode 'Durka Returns' and at the start of chapter 3 you will suddenly see a symbol at the bottom of the screen. If you press ENTER on your remote control while the symbol is visible, you will get to see a short clip of actress Gigi Edgley talking about the action figure for her character Chiana.

Farscape Region 1 Vol. 6: DVD Credits

On the main menu choose extras. Push up on the arrow key and the star/sun above will be highlighted. Push enter and see the DVD credits