The famous secret code if typed in the PC first person shooter game, DOOM, will give you God mode.
The other big code but not really as famous is idkfa which gives you all the ammo in the game and the three coloured key cards.
"What's wrong there Bill? What's wrong there Blair?"
"WE CAN'T PLAY DOOM, WE'VE HAD OUR SHARE!"
"What you need is a code."
"That's right. iddqd will get you out of those problems you have."
"HOORAY! HOORAY FOR IDDQD!"
"That's right kids. You too can succeed in DOOM. Just type iddqd once you enter the game and the rest is MAGIC."
Cheat code needed to activate 'Cena mode' in Doom and also life. No-sell cacodemons, 300 feet falls and exploding cars with ease!
Thank god I hit iddqd before I got DDT'd on the concrete, powerbombed through a table and shot in the head, otherwise I might've had to do a job.