Doom 2

IDDQD          Invincibility
IDKFA          All weapons, All keys, 200% armor
IDFA           Same as above but without the keys
IDCLEV + (episode #) + (level #) = Warp to any episode & level
IDCLIP         Walk through walls
IDDT           While in "map mode" reveals entire map
IDCLIP         Walk through walls
IDBEHOLD + I   Invisibility
 "   "   + S   Berserk pack
 "   "   + V   Invulnerability
 "   "   + R   Radiation suit
 "   "   + L   Light amp goggles
 "   "   + A   Computer area map
IDCHOPPERS     CHAINSAW
IDMYPOS        COORDINATES
IDSPISPOPD     Clipping toggle
IDMUS##        PLAY MUSIC OF A LEVEL

In deathmatch net game, hold Alt and type IDDTIDDTIDDTIDDT to see your opponent on the map screen.