System 6.0  
Code Names:  System 6.0.4:		Antares   System 6.0.5:		Big Deal  System 6.0.6: SixPack 
System 6.0.8: Terminator

Chooser
In chooser version 3.6 click on the version number to see in the User Name box the credits "scott douglass, fjs, jhl".

Control Panels
To see a few credits, click on the version number in the lower left-hand corner of the application.  This applies to the combined control panels where they are all accessed through one application.  The version I checked was 3.3.3.

MultiFinder 1.0 (distributed with System Software prior to 6.0)
Code Names: Juggler, Oggler, Twitcher
Hold down Command and Option while selecting "About Multifinder" from the bottom of the Apple menu.  A scrolling list of credits appears.

MultiFinder 6.0 
Code Names: Juggler, Oggler, Twitcher
Select “About MultiFinder” and leave the dialogue box open for an hour or longer.  If you don’t have the time, patience, or system it says: 
"I want my"
"I want my"
"I want my l--k and f--l"

Original Copy of Multifinder
Code Names: Juggler, Oggler, Twitcher
Leave the “About MultiFinder” box open for about an hour. It will turn into the message “[rot13] V jnag zl, V jnag zl, V jnag zl ybbx naq srry.”

Brad Brown (Zoocat@aol.com) adds this:
The rot 13 means the letters are rotated down the alphabet 13 letters. When you take it back to normal, it says: "I want my, I want my, I want my look and feel."

Zap PRAM
Hold down Command, Option, and Shift while opening the Control Panels to zap the PRAM.