I used cheat codes a lot on Sonic the Hedgehog, I loved playing in debug mode and just building my own levels and things. I can't really think of anything else I used codes on other than maybe Lemmings and passcodes, and Worms. Because Worms was more fun the more destruction you could cause! Oh and I used cheatcodes for second playthroughs of Descent and Descent 2 just to get infinite ammo and stuff.

I also used them on the early GTA games up until Vice City, because lets face it, those games were way more fun that way.

I did get an Action Replay cart for the PSone, but it bricked my PSone. Thought it was just coincidental but I got another PSone and it bricked that too - didn't even get as far as using the thing, as soon as the console was turned on with the cart in the back, bricked.

Command & Conquer: Tiberium Sun was the first game I modded. To get new units and stuff, was pretty dull though.

It's a shame more games don't include these types of codes anymore. Or in the case of Saints Row The Third, they just give you godmode as paid DLC!