These are some codes. Some might just be really secret things that aren't codes, and some will be actual Codes. I will put S or C after the title of the code/cheat/hint etc. S will stand for Secret, meaning something that is not accessed by codes, meaning it's naturally in the game. However, that doesn't mean you don't have to use a code to get to the natural thing. C stands for codes. I don't have lots, but I hope this is enough.
Tony Hawk's Pro Skater 2 (GBA)
Sonic the Hedgehog (Sega Genesis)
Sonic 3 and Knuckles (Sega Genesis)
Sonic Spinball (Sega Genesis)
Super Smash Bros. (Nintendo 64)