Here is a comprehensive list of easter egg codes you can input:

Code: 8487
Result: Outputs a naughty word
Why: Long story short, it's Space Quest 2's fault.

Code: 864285
Result: Outputs "Wilco Must Pay"
Why: Spells out "VOHAUL"

Code: 6978
Result: Outputs an insult
Why: Spells out "MYST"

Also, if you input the correct code before solving the puzzle that gives you the appropriate hint (i.e. you input the code before you're supposed to know what the code is), the keypad will display "CHEAT" before opening the door.