Original Pitfall Mini-Game:
Enter CRANESBABY as a password.
While in this mode, press -
R1 + R2 - Toggles 'Gary Head'
R1 + Circle Toggles 'Elvira Head'
R1 + Triangle on a crocodile screen will make one say 'Hi mom'
L1 + L2 Toggles Infinite Lives

10 Lives:
Enter GIVEMELIFE as a password.

99 Lives:
Enter STEVECRANEME as a password.

View FMV Sequences:
Enter PLAYMOVIES as a password.

View Credits:
Enter CREDITS as a password.

Flying Mode:
Enter ZEROGHARRY as a password.

2D Mode:
Enter 2DHARRY as a password.

Big Head Mode:
Enter BIGHEADHARRY as a password.

Quiet Harry:
Enter STOPTALKING as a password.

View Comic Scenes:
Enter PITFALLCOMIC as a password.

Various Codes:
I don't know what they do, but they are valid codes -
BESTFORLAST
BIGWORMGUY
CAKEWALK
DEEPDARK
DISCOLIGHTS1
GEEHEISBIG
GOINGDOWN
HOTROCKS
JAILBREAK
MAGICGARDEN
MAKEMEMIRA1
METROPILIS
SPOOKYMESAS
TEMPLEME
THUNDERDOMES
VIGILANTE
WOWTHATSHOT