Klapaucius for The Sims. Who cares about cheats for realistic lightsabers and the ability to be turned into chicken? I get to build beautifully furnished houses! [sub]I’m such a dork.[/sub]
You guys have mentioned some classics. My newest favorite is “that’s no moon” in the Clone Campaigns Expansion of Star Wars: Galactic Battlegrounds. It creates the Death Star. Now, there are cheat codes to create an Imperial Star Destroyer and the Rebel blockade runner, and they both have the stats of an air cruiser on the game. I was assuming that the Death Star might be something similar. I got bored playing a mission and tried it out. I floated it across the map to the Gungan base and targeted the Command Center. When it fired, my jaw dropped.
It not only blew up the command center, but every building around it, every unit standing near it, and leveled trees and nova crystal deposits. It’s hardly a whole planet, true, but it’s a stunning display of destruction, nonetheless. My reaction to seeing the blast radius full of rubble and corpses was, “Holy crap! It is the Death Star!” :eek:
Its power is so ridiculously out of proportion to other units in the game that the scenario is as good as over the moment you create it, but it’s still a terribly amusing way to end a game once in a while when you’ve become frustrated or bored.
Especially when you use it against Gungans. 
I like what they did with the chickens in Heretic 2 – the Morph Ovum spell. When used in multiplayer it’d change anyone standing near you, BUT! with a 5% or something chance that they’d become a giant godlike superchicken that could kill you in one peck. Fun!
As for in game cheat codes I’d have to go with the realistic saber combat from Jedi Knight 2. There’s not much more fun than winning a saber duel and seeing the camera do a slow motion spin around a dark jedi while he topples in 2 pieces. Looks even better when you catch them mid jump :).
Sid Meier commented in Rome On 640k A Day (the first Civ strategy guide) that he allowed that because otherwise there would be no contact with other civilizations until the Age of Sail. Yeah, it sucks, but the computer also doesn’t stockpile caravans and dump them all into a city the turn it starts to build a wonder, so you can “cheat” in ways that the computer can’t, without using a cheat code.
I still think Civ1 is better than Civ2 or Civ3. Oh, for the days of using debug as a quick-n-dirty hex editor on Civ1 saved games to give myself all tech or 32767 cash …