I think as retro gamers of many years, we all have encountered along the journey certain games that were either horribly difficult (unfairly so in some cases) or plain extremely flawed for one reason or another. Some of these games would sort of be a "waste" were it not for a code that suddenly makes the game a whole lot more inviting to play.
What is your game and code?
For me, it's Equinox (SNES). The game just punishes you with pixel perfect jumps, death terrains galore, crazy puzzles and a brutal 1 death style system. However, if you go to the title screen and press L, L, R, R, L, L, L, R, R, R, L, L, R, R, L, R... you are granted infinite lives and magic power. It certainly makes the infuriating experience (and flawed gameplay) a lot more "fair" and enjoyable. I use the code mainly for the infinite lives. You can really abuse the unlimited magic powers if you want, though. I try not to, as best as I can.
Typically, I am not a proponent of cheats. I don't look to play a game with cheats enabled, particularly the first time through. But some games, like Equinox, are simply a mutha without it. So, what's your game? Post it and the cheat that makes it (more) playable/enjoyable!
BTW, since we are on the subject of codes, anyone else a big fan of non-password retro games that have a level select cheat? That's my favorite kind of cheat... it increases a game's longevity by quite a bit. And makes the fact that the game lacks a PW or save option a lot more bearable.