Cheats:

Type them in whenever you need them:

idbehold - Displays menu  (followed by S, V, I, R, A, or L for choice):

S=Strength (Berserk)
V=Invulnerability
I=Partial invisibility
A=Full Automap (computer map)
R=Anti-radiation suit
L=Light amplification visors

idchoppers              Gives you the chain saw
idclev                  Warp (followed by episode number and level number)
idclip                  No clipping (you can walk through walls)
iddqd                   Degreelessness mode (God mode)
iddt                    Toggles Automap between normal, full, 
                        and full with objects (enter when in Automap mode)
idfa                    Gives full ammo, 200% armor, and all weapons
idkfa                   Very Happy Ammo 
                        (full ammo, 200% armor, all weapons & keys)
idmus                   Changes the music to a certain mission 
                        (followed by episode and level number)

00 INTROA 01 RUNNIN 02 STALKS 03 COUNTD 04 BETWEE
05 DOOM 06 THE_DA 07 SHAWN 08 DDTBLU 09 IN_CIT
10 DEAD 11 STLKS2 12 THEDA2 13 DOOM2 14 DDTBL2
15 RUNNI2 16 DEAD2 17 STLKS3 18 ROMERO 19 SHAWN2
20 MESSAG 21 COUNT2 22 DDTBL3 23 AMPIE 24 TEHDA3
25 ADRIAN 26 MESSG2 27 ROMER2 28 TENSE 29 SHAWN3
30 OPENIN 31 EVIL 32 ULTIMA 33 READ_M 34 DM2TTL
35 DM2INT

idmypos - Displays your bearing and coordinates in hex