Activate the console using Ctrl+Alt+Tilde (~). Now enter this command, so you
can open it by just pressing Tilde in the future:

com_allowConsole 1

Now use any of these codes. Please note that you can turn on this using "1"
and turning it off with "0" (showFPS, pm_thirdperson etc.):

aviDemo = Make an AVI out of a Demo-file
benchmark = Game Benchmark
com_showfps 1 = Show FPS
doomhell = Skip all levels to last
editor = Open Editor
freeze = Freeze all on screen
g_nightmare 1 = Unlock Nightmare-mode
g_showplayershadow 1 = See your shadow during Single-Player
gfxinfo = Graphics card information
give all = Full load of weapons, armor and ammo
give ammo = Max ammo
give armor = Max armor
give berserk = Berserk
give health = Max health
give invis = Invisibility
give keys = Acquire all the keys you'll ever need
god = God mode
kill = Commit suicide
killmonsters = Kill all monsters
noclip = No Clip
notarget = Invisible to most enemies
playdemo demo_name = Play a Demo
playdemo stop = Stop recording a Demo
pm_crouchspeed ### = Define how fast you can crouch
pm_runspeed ### = Define how fast you can run
pm_walkspeed ### = Define how fast you can walk
pm_noclipspeed ### = Define how fast you can navigate using the noclip-cheat
pm_jumpheight ### = Define how high you can jump
pm_thirdperson 1 = 3rd Person-view on
quit = Quit
recorddemo demo_name = Record a Demo
set g_dragEntity 1 = Grab objects/bodies when you fire at them
status = Your game status

Type6 submitted that's it's possible to find every code in the game by
pressing a letter (A, for example) and pressing TAB.