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