IDDQD-This gives you a permanent God Mode. God Mode is the same as being invincible. Nothing can hurt you! Not even toxic waste. I like to use this code mostly for trying to pass a level on fists, which is lots of fun. I'm sure you'll find your own ways for using this code.
IDDT (Automap only)-If you type this code once it will show the entire map of the level. But if you type it twice you also get radar! I find the radar hard to read though. It shows all of the objects and the enemys! What really gets me is that it also shows the enemys you already killed!
IDKFA-If you type this in you get all weapons and ammo. I find this code useful when trying to have fun on an easy level. I found that by typing IDFA you get all weapons and ammo without the keys. I usually use IDFA because that way you have all the weapons and ammo but you get to play the whole level.
IDBEHOLD
V-Invulnerability Artifact.
S-Beserk Pack.
I-Partial Invisibility Artifact.
R-Radiation Suit.
A-Computer Map.
L-Light-Amplification Goggles.
All I have to say about the IDBEHOLDs is that "If you want something, get it!"
IDSPISPOPD-This code allows you to walk through walls. If you walk to where there isn't any level and the screen looks funky, don't panic! Just look at your computer map and head towards where the level is. Thank God, in Doom2 they shortened this code to IDCLIP. I think that was a great idea, don't you?
IDCLEV (episode#) (level#)-This is obviously the level select. But in Doom2 you should type IDCLEV then just a level number as in: 01 or 23. (In Doom2 you must use two digits.)
IDCHOPPERS-This code just gives you the Chainsaw.
IDMYPOS-I never really used this code. I find it unnecessary, but you might find a good use for it. Well, it displays your x,y coordinates in hex code or something. I'm really not sure!