Doom 64
Cheat Menu
For access to a hidden cheat menu ("Features"), enter the password "?TJL BDFW BFGV JVVB". Start the game, then press Start and select "Features." From here you can choose from level select, invulnerability, all weapons, and more!
Credit Goes To:Chris Divine ( via IGN 64

 Cheat Menu
 Funny Messages
 Start Game w/

 Ultimate Password
 View Demo of
 Level 32

Funny Messages
After you die, notice that you fall but the screen won't go "blood red" until you press a button. If you wait and don't press anything, the messages at the top will say stuff like "Look at those demon feet!"
Credit Goes via IGN 64

Start a New Game with Everything
These passwords will start a new game with 200% health, 200% armor, all weapons, a backpack, full ammo for all weapons, and all three Demon Artifacts.

Difficulty     Password
BE GENTLE!     ?qdm 7hyc bb4x fjvb
BRING IT ON!   ?ld4 7hyb bb4d cjvb
I OWN DOOM!    ?gfm 7hyt bb9x khvb
WATCH ME DIE!  ?bf4 7hys bb9d hhvb
Credit Goes To:Ledmeister and Tom Freeland 5 via IGN 64

Ultimate Password
This password takes you to The Absolution with a three-beam version of the alien weapon, and all the keys you need to unlock the special switches on this level. For all this and more, enter the password "W93M 7H20 BCY0 PSVB".
Credit Goes To:Cloud Strife and Michael Thompson via IGN 64

View Demo of Level 32
At the password screen enter: RVNH 3CT1 CD3M 0???. This will add a fourth demo which shows how to beat level 32.