Press the "~" key (next to "1") during the game and enter the following commands in the console window:

"GOD" - Invincibility ON/OFF
"GIVE 2" - Activates Weapon 2
"GIVE 3" - Activates Weapon 3
"GIVE 4" - Activates Weapon 4
"GIVE 5" - Activates Weapon 5
"GIVE 6" - Activates Weapon 6
"GIVE 7" - Activates Weapon 7
"GIVE 8" - Activates Weapon 8
"GIVE S 255" - 255 Shotgun Shells
"GIVE C 255" - Battery Charged
"GIVE R 255" - 255 Rockets
"GIVE N 255" - 255 Nails
"GIVE H 999" - Health to 999
"IMPULSE 9" - All Weapons and Ammo
"IMPULSE 11" - One Rune each; can be used 4 times
"IMPULSE 255" - Activates Quad Damage
"FLY" - Flight Mode ON/OFF
"R_FULLBRIGHT x" - Set Brightness (x = a number, e.g. 10)
"NOCLIP" - Clipping OFF/ON, you can walk through walls
"MAP ExMy" - Go to Map x and y = enter numbers
"NOTARGET" - Invisible/Visible
"MAP START" - Jump to Start
"MAP END" - Jump to Final Boss
"CROSSHAIR X" - Crosshair

Close the console window by pressing the same key (~) again.