Insert the Side/Disc that contains the feature presentation and from the Disc's Main Menu, select "Special Edition" which brings up a dedicated Menu to access the film's Special Edition Cut. On the right hand side of the panel you will notice some weird, unreadable symbols that will give you access to the hidden feature - actually they are roman numbers. Take your remote control and press the key combination "82997" - the date of Judgment Day in the movie - and the symbols on the side should change to the text "The Future Is Not Set". If they don't, please make sure to press the "Enter" key on your remote control after every one of the digits. Once correctly entered, the eyes of the endoskeleton will light up and you will see a new menu entry appearing, giving you access to the "Extended Special Edition" Version of the movie with the alternate ending and other scenes integrated in the film.
Insert the Second Side/Disc of the DVD and bring up the main menu. Don't touch any buttons, and every 30 seconds, the Disc cycles through one of three different 'breaks' - in the first the lower left hand corner flickers with lightning and the words "Join The Resistance" appear. Select it and you will find a Weblink that takes you to Artisan's Website. The second break is a small head of Beethoven, which will take you to the original Swelltone Trailer, and the most impressive, third break comes in the shape of the T-1000. His head will morph out of the center of the screen and say "Get Out!"
All easter eggs were generously submitted by Bjorn_K