Enter the following at the "Enter Password" screen
FREEPLAY - Level select
GIMME - All weapons
.IMHUGE - Big mode
NODIE - Invincibility
NOSEEUM - Invisibility
SHORTY - Mini mode
THDOTEST - Debug information
Level Passwords
Boot Camp BOOTCAMP
Dinner Table DINNER
Bridge OVERPASS
Refrigerator COOLER
Graveyard NECROPOLIS
Castle CITADEL
Tan Base MOUSE
Revenge ESCAPE
Desk ESCRITOIRE
Bed COT
Plasticville BLUEBLUES
Toy Shelf BUYME
Cashier EXPRESS
Toy Train Town LITTLEPEOPLE
Rocket Base NUKEM
Pool Table EIGHTBALL
Pinball Machine BLACKKNIGHT