While playing, press CTRL + ALT + ~ to bring up the console. Then, enter one of the following codes:

Effect - Code

All Weapons, Ammo, and Armor - give all
All keys for the current level - give keys
Fills your health points to the maximum - give health
Fills your armor points to the maximum - give armor
Fills your ammo to the maximum - give ammo
Gives you the Grabber - give weapon_grabber
Gives you the Shotgun - give weapon_shotgun
Gives you the Machine Gun - give weapon_machinegun
Gives you the Chaingun - give weapon_chaingun
Gives you Grenades - give ammo_grenade_small
Gives you the Plasma Rifle - give weapon_plasmagun
Gives you the Rocket Launcher - give weapon_rocketlauncher
Gives you the BFG 9000 - give weapon_bfg
Get the new grabber gun - give weapon_grabber
Get the new double-barrel shotgun - give weapon_shotgun_double
Invincibility - god
Spawn a hazmat zombie - spawn monster_zombie_hazmat
Spawn a "Forgotten" skull monster - spawn monster_flying_forgotten
Another "Forgotten" spawn code - spawn monster_flying_forgotten2
Spawn a vulgar demon - spawn monster_demon_vulgar
Spawn a bruiser demon - spawn monster_demon_d3xp_bruiser
Spawn the first of three hunter demons - spawn monster_hunter_helltime
Spawn the second of three hunter demons - spawn monster_hunter_berzerk
Spawn the third and final hunter demon - spawn monster_hunter_invul
Start a new game on Erebus Level # (1 to 6) - map game/erebus#
Start a new game in Phobos Sector # (1 to 4) - map game/phobos#
Start a new game in Delta Labs - Unknown - map game/deltax
Start a new game on the final Hell level - map game/hell