Well the hidden feature on Memento isn't exactly very well hidden, but here it is anyway. To find the version of the film that plays in chronological order, head to the "Special Features" menu and wait till Memento Mori appears and begins to fade. Just after the polaroid fades, push your 'enter' button and you should access the secret feature with no problem. Hey presto!
There is another hidden feature tucked away in the 'Website' portion of the "Special Features" menu. When you enter the 'Website' page, navigate right with you remote until you highlight the word 'Questions'. You will be presented with a screen similar to the Memento web site, asking the question 'Who Did I Kill?'. Click on the arrow and you will be presented with a list of names. Correctly identify the name of Leonard's victim to see the DVD credits (it helps if you've seen the film first).
All easter eggs were generously submitted by SirDonk