In some versions of DOOM, try pressing -IDDT in map mode while 
playing a DM game...

Here are cheats to type while playing the game:

Code        What it does 
~~~~        ~~~~~~~~~~~~
IDFA        [v1.666+] All the ammo, 200% armor, no keys
IDBEHOLDR   radiation suit
  "  "  I   inviso power up
  "  "  V   invincible power up
  "  "  A   computer map
  "  "  L   light amplification power up
  "  "  S   bezerker power up
IDCHOPPERS  get chainsaw, invincible, and prints "Doesn't suck - GM"
IDCLEVxy    warp to episode x (1-3) level y (1-9) 
IDDQD       Degreelessness mode (God mode)
IDKFA       Very Happy Ammo (full ammo, 200% armour, 199% health, all keys)
IDMYPOS     prints your co-ordinates in hex
IDSPISPOPD  No clipping, walk through walls
IDDT        [Map Mode] toggle map detail: