Passwords:
Deimos Lab: pk04tmkstr
Refinery: h3bwgbdldb
A very diffucult level: v8hfn54356
Club DOOM extra level code: JCGDNSL556

Invincibility:
PAUSE GAME, DOWN, L2, SQUARE, R1, RRIGHT, L1, LEFT, CIRCLE

All weapons and ammo:
PAUSE GAME, X, TRIANGLE, L1, UP, DOWN, R2, LEFT, LEFT

All the maps plus objects:
Triangle, Triangle, L2, R2, L2, R2, Right, Circle

Free Stuff:
X, Triangle, L1, Up, Down, R2, Left, Left

Level Warp (Unverified):
Right, Left, R2, R1, Triangle, L1, Circle, X

X-Ray Vision:
L1, R2, L2, R1, Right, Triangle, X, Right

Transparent code: L1,R2,L2,R1,RIGHT,TRIANGLE,X,RIGHT

Multiple level passwords:
At enter password screen hit X to select B, but keep holding it. Move the 
cursor to the right one space at a time. It will look like this "BCCDDFFGGH" 
By moving one space at a time, in different directions, you can find 
many passwords. For example club doom is down, right, down, right, 
down, right.

If you die on a level and you had collected a few weapons previously, rather 
than restarting the level by pressing a key after you die, press START then 
SELECT to get the options screem, go to PASSWORD and 
press X twice. This will restart your level with all previous weapons and 
health.

If you can make it all the way to level 30 without dying, you will get to play 
ping pong with one of the DOOM 
creatures.