IDDQD- God Mode. Gives you invulnerability.
IDKFA- Equips you with all the weapons and keys for the doors. Also gives you full ammo on each weapon. Once you begin to run low on ammo type IDKFA again to replenish your ammo supply.
IDCLIP - Turns no clipping mode off. This allows you to walk through walls.
IDCLEV##- Usage: IDCLEV the number of the level you want to go to. Example: IDCLEV30 warps you to the 30th level.
IDBEHOLD - This gives you certain special items which you may need to use in the game. The items are: Invisibility, Radiation Suit, Lite-amp, and more.

Secrets:

Secret Wolf Levels- In Doom2, ID has put in two
secret wolfenstein 3-d levels with wall textures
from wolf3-d and some enemies from it also. Both
levels are located as maps 31 and 32. How to access the levels: To get to the wolf levels there is a secret entrance in either level 15 or 16 to access the wolf levels. Easier than that is to use IDCLEV31 and warp straight to the Wolfenstein levels. Level 31 is shaped exactly as the first board in Wolfenstein and contains the entrance to the secret level 32. Level 32 is shaped as the last level of the game in Wolfenstein. The boss is the cyberdemon. Once you complete level 32 you will be warped back to level 16.

Hints:

Beating the last guy- The last guy in the game is
a big skull with a hole in its head exposing its
brain. Through its brain it fires stuff out which
resurrects dead demons on the board (There are many of them there). To beat the monster, you have to move up to the top platform and activate this skull wall so that a tower rises in front of the end boss. Next go down and get on the tower by pressing the space-bar on it and lowering it. Get on top of it and ride it upwards, firing at the end boss trying to hit his exposed brain. Also, remember to take out the demon firing rockets just below the skull.

This board is almost impossible to beat without using god mode (IDDQD)