The Cheatmodes

There are two different types of cheats:
1. Commandline-parameters, which have to be typed in, when starting DOOM from DOS.
2. Online-codes, which have to be typed "blindly" (and quickly) while playing.

Zu 1. (Commandline-Parameters) 
-DEVPARM = Developer-Mode. Pressing [F1] during the game will produce a screenshot in PCX-format. Slows down gameplay a little.
-WART xy = Gives you a lift (warp) to episode x mission y.
-NET x = Starts a NETWORK-game for x players
-RECORD name = Records everything you do inside DOOM as a film (*.LMP) with the specified filename.
-PLAYDEMO name = Plays back your recorded *.LMP-files.
-FILE = Inserts a patched *.WAD-file (PWAD, *.WAD). You can specify whichever path you wish, thus you don't have to keep those hundreds of PWADS in your DOOM-directory.
-NOMONSTER = A game without monsters (what for?)

ONLY FOR DOOM II:
-TURBO xxx = Speed-Boost for your player (even in multiplayer-mode). XXX represents any number between 0 and 250.

Parameters may be cued too, eg.
Doom -devparm -nomonsters -file E:\EPISODE\xy.wad


Zu 2. (Online-Cheats)
IDBEHOLD = activates a multiple-choice menu, another letter has to be provided
A - complete map, "unknown" areas are displayed in decent gray.
R - radiation-suit
I - partial invisibility
V - amplification-visors (MORE LIGHT!)
S - go berserk!
L - magic light, means that lights are set to full throttle...

IDCLEVxy = warp to episode x mission y

IDKFA = "very happy ammo", ALL the weapons, keycards and lots of ammunition...

IDCHOPPERS = just what you guessed, the famous CHOPPERS!

IDDQD = "Degreelessness- or GOD-Mode", invulnerability

IDDT = (works only in MAP-mode) complete map. When typed twice, all objects and monsters are visible as green triangles.

IDSPISPOPD = "no clipping"-mode, walk thru walls, doors and Cacodemons...