PowerBook:         
 
            AutoRemounter
            Click on the version number.

Control Strip
In the Control Strip control panel, Option click the version number.

Elvis
Option-click the version number of the PowerBook Control Panel (version 7.2 or later) then repeatedly push the Option key.  He’s there!
 
           Original Code Names
        Turn on Balloon Help, press Caps Lock, and point to the up arrow in the menu bar.  It will say “This file allows your Macintosh TIM or Derringer to display an icon..." (those are original names of the Powerbook.  This is any more of an Easter Egg then a mistake given that the extension was not modified before System 7.0.1 was released.

PowerBook Display Control Panel
Click on the version number.

PowerBook File Assistant
Hold down Option when going to "About PowerBook File Assistant...".  Keeping holding it down and look just to the right of the close button.  These are various names of the developers for the File Assistant rapidly shuffling in order.  Once you let go of Option you will land on a name.  This is one of the most interesting eggs I have ever seen.

PowerBook Setup
To see some credits push Option and click on the version number of the PowerBook Setup Control Panel.