Crap games like Stolen or Death By Degrees, yeah I'll cheat. No point playing fair against poorly designed games. Classics like Mario and Metal Gear? No way!! You have to savor a game like that.

Konami Code was used for Contra.
I've used some cheats to unlock bonus stuff.
I needed a map for Legend of Zelda (NES).
Sometimes I use guides for sidequests on RPGs.
I try to go as long as possible without it though, and almost never cheat on the core part of the game. Only when I am truly stuck do I do it.

Always, even in Brain Training.

It defeats the purpose to cheat at single player games. But online games cheating is necessary to help you win against opponents. Cheating gives you a winning edge online for games such as COD 4, Halo 3, Gears of War, etc.
I do not play games online, I do not have enough time to burn.

I don't cheat. Ever.

I'll only cheat by pulling out the game genie for those really old games that are impossible.  If anyone has tried to beat hylide for the nes they'ed understand.

I will definitely cheat. A good example was the flame in Zelda PH. I would have never figured out you're supposed blow into the DS. I just don't see any way I could have guessed you're supposed to do that.

Maybe it was in the manual? I'll check the walkthrough before the manual LOL.

Second time going through a game, get 100% using faqs/walkthroughs or when I'm really really stuck I look up online help, faqs/walkthroughs.

Use money cheats in Sims to build stuff I have in mind that requires more than the little 20,000 they give for a familly. Also use some advanced building cheats like floor elevation, roof angles, 45 degrees of angle objects and stuff like that.

I try to avoid walk thoughts and cheats because once I use it once in a game I'm usually unable to stop. I start thinking what will I miss if I don't and then instead of playing the game I'm following instructions in a walk through.

I havent cheated in a long time in Multiplayer or Single player. When I had a Nintendo 64 I cheated all the time in MP and SP. For Single player, right when I got a game I'd look for codes like the 'unlock everything' code. For Multiplayer the N64 was only split-screen so I definitely cheated by giving the person next to me a 'wet willy', 'purple nurple' and 'dead legs'. But cheating is not my style anymore :)