System 6.0:    System 6.0.4:		"Antares"        System 6.0.5:		"Big Deal"
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 for the combined control panels where they are all accessed in one application.  The version I checked was 3.3.3.

Happy New Year!
In system 6.0.7J (kanjitalk) set the clock to 1/1/92 and restart to see “Happy New Year” in Japanese.  (I believe it has to be a Japanese computer, but you can try it!)

MultiFinder "Juggler, 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
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.”

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

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