The Grand Theft Auto series was gonna appear on this list no matter what; the cheats are just too plentiful and too powerful to ignore. Press a few buttons and the cars fly... or turn pink... or go invisible. Press a few more buttons and the city's citizens become prostitutes... or ninjas... or fast food employees... or Elvis impersonators.
The combination of cheats we've chosen to highlight, though, is more than a crazy diversion - it could actually help you win missions and beat the game. How? Multiplication. Increase your strength by five, by ten or by however many random strangers you can attract to your cause. Since the game is now programming them to follow you, they shouldn't require much coercion.
To form your very own pedestrian army, you need two codes: "Pedestrians Have Weapons" and, depending on the game, either "Pedestrians Follow You," "Attract Followers" or "Recruit Anyone." Visit CheatPlanet for the specifics on your platform and your chosen GTA.
Oh, and did we mention what else a bunch of devoted groupies is good for? This.
The ability to walk through walls doesn't do much in the open, sandbox world of Grand Theft Auto, but it works wonders in the tight and twisty world of Doom. Type "idclip" and you're unstoppable. Can't find the red key? Stroll through the red door without it. Can't escape the big demon-headed thing spitting fireballs? Hide outside the borders of the map until you catch your breath.
Next up... nudity! And censorship!