Perfect Head
"Perfect Head?  What?" you're probably saying.  Perfect head is the ability to take a picture of your face using the Game Boy Camera, and stick that face onto a character in multi player, or the enemies in single player.  Unfortunatly, it IS NOT IN THE GAME.  It was taken out in February, because of some issues.  Think about it.  If one psycho kid brought a gun to school, and they found out he was playing Perfect Dark with his classmate's heads as the enemies, what would happen?  Huh?  So Rare made a smart move and took it out.  But think of how cool it would have been!  These pictures are all that remain of Perfect Head.
Click on that image for a bigger one, that you can read better.  It shows the words "Perfect Head" in the main menu. 
Here is how you would access Perfect Head.  A blank face, which would be filled with your own, or whoever's you scanned.
Here is the picture being downloaded into the game after it was taken with the Game Boy Camera. 
Here is one of the faces that was taken with the Game Boy Camera.  This is the last step, after the face was downloaded into the game.
Here's the contraption that did it.  The Game Boy Camera, connected to the N64 controller through the Transfer Pack.