2. Debug Mode - Sonic the Hedgehog 2
How It's Activated: Go to the Sound Test menu, play tracks 19, 65, 09, 17, and then return to the title screen and press A and Start. Then play tracks 01, 09, 09, 02, 01, 01, 02, 04.
What It Does: It unlocks Debug Mode, which allows you to basically mess around with the game's code, throwing items all over the place, phasing through walls and even transforming yourself into the game's objects. Though plenty of games now allow you to operate Debug Mode through console command lines and so on, Sonic 2 was the first major game to offer it, and to present such a game-changing experience as it did so. You could up the difficulty by lining the maps with more enemies, skip whole stages entirely, or even give yourself a ridiculous number of rings. Also, Debug Mode houses items that the developers took out of the game at the last minute yet didn't delete the code for, and really, this feels a lot more like God Mode than Doom ever did.