Apple PowerBook 

Control Panels
            
            AutoRemounter
            Option-click on the version number in version 1.2.

Control Strip
In the Control Strip Control Panel (version 1.1 and others), Option click the version number to see the author. 

In Control Strip, version 1.3, hold down the option key while clicking in the "Current hot key" box.  This will change the name in the title bar to the author of the control panel, "Steve Christensen."  From: David Kwinter (dkwinter@direct.ca)

PowerBook
Option-click the version number of the PowerBook Control Panel (version 7.2 or earlier) then repeatedly push the Option key.  He’s there!  The Elvis trick doesn't work in Powerbook Control Panel 7.3.1.

            Caps Lock - 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 not any more of an Easter Egg then a mistake given that the extension was not modified before System 7.0.1 was released.

Portable
In the Portable control panel, Option-clicking on the version number 
brings up credits.

PowerBook Display
Click on the version number in older versions.  In version 1.1 balloon help the version number.

PowerBook Setup
To see some credits push Option and click on the version number of the PowerBook Setup Control Panel.  Works in version 7.3.1.

PowerBook  Applications
PowerBook File Assistant
Hold down Option when going to "About PowerBook File Assistant...".  Keep 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.

              The Apple Portable
Macintosh Portable Code Name: Laguna, Riveria, Malibu, Esprit, Guiness
Macintosh Portable (with backlit display) Code Name: Aruba, Love Shack, Mulligan
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Remember that portable thingy released a long time ago that was about etwice as large as today's LCs.  No?  Well, I don't either, and was surprised when I found out about its very existence.  Here are a couple eggs for anyone (I can't imagine who) that still has a Mac Portable.

Kevin Schoedel (kevin@quipu.tdkcs.waterloo.on.ca) tells me the hardware etch, where the proprammer's signatures are etched in raised lettering, is also true of the Mac Portable, released 2 years after the SE. The names are inside the bottom (base) of the case. There are 62 signatures (if I found them all, and counted correctly), plus 7 names in 'type' under the heading "Product Design Team".