idkfa- You get max ammo, max armour, max weapons and all the keys. This cheat is more effective if you've found a backpack (if you have a backpack, then using the cheat gets you twice as much ammo as without a backpack).
idfa- same as
idkfa, but without the keys (version 1.666+ only)
iddqd- god mode. make you invincible
idclev x y- warps you to episode x, mission y
idmypos- shows your position in doom (in hex)
idbehold- allows you to get more of different items depending on which key you press immediatelly after the code:
idspispopd- Walk through walls (zero clipping, they call it). Type this again to turn it off. You can't activate teleporters, pick up things, etc., when this mdoe is on. Plus, it can be tricky to fight the bad guys because their gun blasts, etc. can knock you through a wall!).
idchoppers- gives you the chainsaw
iddt- In the Map mode (after pressing the TAB key)