It's really tough to come up with a valid excuse for cheating, or to justify cheating as "not that bad." Unless, of course, you're talking about video games. Cheating has been a part of video games since the beginning, and in a lot of cases, developers have encouraged it by hiding fun modes, extras, and Easter eggs in games that are literally only accessible via some form of cheating.
Beyond that, old school games were often just too hard, sometimes cheaply so. In order to mitigate this, developers included cheat codes that would alleviate a game's difficulty in some manner. Other times, gamers would find ways to cheat in games that weren't planned by the people who made them, but we still did it anyway because, well... sometimes games are just better when you cheat. If the choice is between cheating to keep enjoying a game or playing it honestly and rage quitting it forever, the correct option seems pretty obvious.
20 Grand Theft Auto III
Grand Theft Auto III probably stretches the definition of "classic" a bit in terms of its age, but it's hard to deny its standing as a classic in every other sense of the word. Subsequent GTA games had much better stories and gameplay variety, but GTA III was largely just a playground for mayhem tied together with a cliched and forgettable story. With that in mind, it was best enjoyed with cheat codes that allowed us to skip the boring bits and go straight to having tanks and rocket launchers with which to wreak maximum havoc in minimal time.
19 Mortal Kombat (Sega Versions)
Nobody would've given Mortal Kombat the time of day with Street Fighter II in the building if it weren't for the blood and fatalities. Being able to bring MK home was exciting for gamers at the time... until they discovered that the home versions cut out the blood and severely tamed the finishing moves. That is, unless they were playing one of the versions that came to Sega's home platforms, where a cheat code could be entered that unlocked all the R-rated goodness of the arcade original.
18 Teenage Mutant Ninja Turtles
In a world before the iconic Teenage Mutant Ninja Turtles arcade game existed, all we had was the wonky NES game— so we learned to love it, warts and all. However, the thing that made that forced tolerance most difficult was the game's extreme difficulty and notoriously cheap design. The worst offender was the infamous dam level, which combined finicky controls with electrified seaweed you couldn't so much as graze without it damaging you. Enter a handy Game Genie code that renders the seaweed harmless, however, and make the game's worst stage a cakewalk.
Metroid was one of the first games to feature a large, open world that you could explore in four directions, but that exploration was almost completely blind, requiring exhaustive trial and error, and literally combing every inch of the environment. Fortunately, the game's password system allowed for players to help themselves to better weapons and higher health bars right off the bat— and, thanks to the famous "Justin Bailey" code, they could explore Metroid's impressive world fully powered-up (and with a suit-less Samus).
16 Final Fantasy VII
The Final Fantasy series has always been full of items and objectives that can only be unlocked via obtuse and random parameters. There is almost no way to 100% most of those games without cheating via walkthroughs. Final Fantasy VII was a particularly egregious offender of this trope, with some of the game's most powerful and important items nearly impossible to get without either cheating, or stumbling upon them by complete accident. If you claim you obtained a Gold Chocobo entirely on your own with zero help, you're definitely lying. To say nothing of the re-release's literal built-in cheat codes.
15 Donkey Kong Country
While Sonic won a lot of gamers over to the Genesis side of the 16-bit console war, Donkey Kong Country stole most of them right back with its (for the time) mind-blowing visuals. But DKC was not a game to be taken lightly, and could get downright punishing— not helped by the cheap mine cart stages or having to replay multiple levels if you ran out of lives. Enter a handy 50-life code that helped to level the playing field of this fun, but sometimes unfair, platformer.
14 SimCity 2000
The SimCity games are realistic almost to a fault depending on how much you know and/or are willing to learn about city planning. They are most satisfying for the people who put in the time and just can't get enough of micromanagement. Of course, the average gamer just wants a city to mess around with (and destroy in spectacular ways), which is where cheat codes come in that grant things like unlimited money. SimCity 2000's most famous/useful cheat literally involves typing out "imacheat," giving you a couple of seconds of shame before you have hours and hours of fun.
13 Sonic The Hedgehog 2
The meta game in the early Sonic the Hedgehog games was to collect seven Chaos Emeralds by the end of the game, which not only earned the true ending, but allowed you to transform into "Super Sonic." Unfortunately, Tails made this goal nearly impossible in Sonic 2 for inexperienced players, lagging a half-second behind Sonic in the bonus stages, often preventing you from getting the necessary number of rings to get an emerald. But everyone should still have a chance to enjoy the speed and the power of playing as Super Sonic, which a Game Genie code allows even without having all the emeralds.
12 NBA Jam
Does it still count as "cheating" if a game is built around inputting cheat codes, so much so that the method of entering them is prominently featured in the game? Maybe not, but as the cheats in NBA Jam are still referred to as such, we're including them. From Big Head Mode, to being able to perform full-court dunks, to unlocking celebrities and politicians as players, NBA Jam reveled in its many novelty cheat codes that made an already fun and ridiculous game that much more delightfully silly.
11 Mega Man
The Mega Man games have always been known for their unforgiving difficulty, and that started with the very first entry in the series. One of the most common ways to cheat at Mega Man is to look up the proper order with which to tackle the levels in order to make progression as easy as possible— it saves a lot of frustrating trial-and-error in trying to figure that out yourself. But there are other ways to cheat at Mega Man that make the game a little more fair, such as the pause trick that makes the rage-inducing Yellow Devil boss fight a (time-consuming) breeze.
What seemed like a harmless tutorial ended up being the bane of millions of gamers' existence: the infamous "Driving test" in the PlayStation classic Driver. Even people who eventually beat the game and became quite skilled at it struggled when they started over and had to do the punishing test again. It's easy to imagine that a lot of people simply quit when they couldn't pass the test, and missed out on of the the PS1's best games. Enter the GameShark cheat device, skip the test entirely, and get right into the first (and much, much easier) real mission.
The PC gaming scene of the early-'90s revolutionized the community and the industry as a whole in countless ways, and many of the most important contributions are wrapped up in a little game called Doom. But we're not going to get into all that— we're just going to talk about how fun it was (and still is) to mow down demons with rocket launchers and slice them up with chainsaws. And who wants to actually find and unlock such weapons when cheat codes can put them in your hands from the very start?
8 Street Fighter II (SNES Version)
It's easy to forget that the version of Street Fighter II that featured playable boss characters and allowed for mirror matches wasn't actually the original. Retroactively referred to as "vanilla SFII," that version even came to the SNES. And while there is no way to make Balrog, Vega, Sagat, and M. Bison playable in that version, a cheat code did bring the mirror matches of Champion Edition into that version, which was nice for those that couldn't afford to drop another $70 on a second version of SFII right away and could at least get one of the defining features of later editions in the game they already had.
7 Zelda II: The Adventure Of Link
Zelda II: The Adventure of Link is often considered the black sheep of the Zelda franchise, in large part dude to its high difficulty and unusually vague structure. To be fair, the first Zelda was all about blindly exploring a big open world as well, but you were guaranteed to progress as long as you burned every bush and bombed every wall. Not so for Zelda II, which contained a number of hidden items and paths that only blind luck could get you to without cheating by consulting a walkthrough.
6 Super Ghouls 'N Ghosts
The Ghosts 'n Goblins/Ghouls 'n Ghosts series— as well as their spiritual spin-offs, the Maximo games— are some of the most brutally difficult games of all time. It's not unusual for even the most seasoned of gamers to lose their armor and their life within literal seconds after starting one of the games. Super Ghouls 'n Ghosts was at least merciful enough to include a level select cheat code which allowed you to see all of the game's many cool levels (and their great tunes)— if only for the first few seconds.
5 Ikari Warriors
A major pioneer of both run-n-gun action games as well as co-op games in general, Ikari Warriors is as ridiculously hard as the many games it inspired. Even with a friend, be prepared to lose a lot of lives very quickly. Fortunately, if you play the NES version, you have a few cheat options that'll level the playing field with extra lives and continues. And you will most definitely need them if you want to actually have fun with this groundbreaking but brutal classic.
If just seeing the above screenshot gave you PTSD, you're not alone— the "turbo tunnel" stage in Battletoads resulted in a lot of punched pillows, thrown controllers, and obscenity-spewing children. What's even more messed up is that the turbo tunnel isn't even halfway through the game, and it only gets progressively harder from there. Needless to say, cheat codes that gave you extra lives in Battletoads were a must if you wanted to actually enjoy the game... let alone see the 75% of it that comes after that stupid tunnel.
3 Kid Icarus
Most former NES kids have really fond memories of playing Kid Icarus— especially the ones that edited out the part where the game was punishingly and unfairly difficult. To be honest, the game really isn't all that good objectively-speaking, but it does have some memorable songs and neat level design that are worth experiencing... but only with one of the many available cheat codes that make it somewhat bearable to actually play.
Up, up, down, down, left, right, left, right, B, A, Start. What else is there to say? The reason that code has become its own pop culture touchstone is because an entire generation tapped that out each and every time they loaded up the NES classic Contra, not even considering the option of trying to start the game without doing so. The code gave players 3o lives, and turned a game about inching along and memorizing patterns into an all-out action movie-style romp that was the centerpiece for thousands of sleepovers.
1 Castlevania II: Simon's Quest
There are games that try to misdirect you in creative ways, nudging you toward the wrong path if you aren't paying attention and reading between the lines. Castlevania II is not one of those games. Instead, the game is notorious for having NPCs that literally lie to you about where you need to go and what you need to do next, while also requiring you to do completely inane and arbitrary things in order to progress. When a game itself is cheating, it's only fair to cheat back, right?
Sources: Prima Games, Neoseeker, Game FAQs, Cheat Code Central