Cheat Codes
              (The only cheats that work in Nightmare mode are IDDT and IDCLEV)

IDBEHOLD +
I: Invisibility (1 minute)
S: Berserk (1 level)
V: Invincibility (30 seconds)
R: Radiation Suit (1 minute)
A: Computer Map (1 level)
L: Light Amplification Goggles (2 minutes)

IDKFA: Adds all weapons, ammo, and keys.

IDFA: Gives you ammo and weapons, but not the keys.

IDDQD: Makes you invincible.

IDCHOPPERS: Gives you the Chainsaw.

IDCLIP: Enables you to walk through walls and walk up to high levels.

IDCLEV + 1-32: warps you to selected level (33-39 Crashes Doom!).

IDMYPOS: Shows your coordinates on the map.

IDMUS + 
Any level number changes the music to the music of that level.
33: Storyline
34: Theme
35: Intermission

IN MAP (some of these aren't exactly cheats)

IDDT: When typed once adds walls to map; when typed twice adds objects.

K: Toggles follow mode.
+/-: Zoom In/Out
G: Grid
M: Marks your position.
C: Clears all marks.
0: (zero, not "Oh") Toggles between zoomed and full view.