The Quality Assurance tool that became an Easter Egg.

Cheat codes.

To most people, cheat codes are special input sequences that are not explicitly explained in a video game's instructions that give the player access to special items or power ups in game. Usually, it's to boost a player and give them incredible advantages, but sometimes it's something as simple as a cosmetic change or access to a different sound or difficulty level. It's assumed that cheat codes are accidental glitches made by the developer and the player is just taking advantage of the glitch. 

Some games had invisible tunnels, which led many to believe they were glitches in the game.

This is so far from the truth. 

Cheat codes are, in fact, carefully programmed into a game. It's done on purpose as a tool for Quality Assurance (QA or game testers) to access certain parts of the game that need to be tested so that the game can ultimately run smoothly. These codes are revealed (or even unlocked) sometime after the game is released as a way to give more longevity to a game. 

Back in the early 70's and 80's, games had no way of being patched (or fixed) via internet because there was no internet at the time. Games were run off of cartridges and had no lasting memory except for the hard coding used to make the game. As such, games needed to be as bug-free as possible and the only real way of testing games was to actually play through the game. Consequently, it became difficult for testers to adequately test latter parts of a game as it took time to get to those levels or the player was eliminated before they could get there. Hence, the programmers installed cheat codes so that the testers could either jump levels or access power ups to test various aspects of the game. Many cheat codes were never uninstalled before a launch of a game for fear that removing them would affect the game itself. Since they were hard coded into the game, players just assumed they were accidents made by the developers.

Nowadays, with the advent of the internet, hard drives, and editable media, many cheat codes and tools that QA use can either be shut off or disabled and legitimate glitches/bugs in-game can be fixed via internet even after a game has released. Cheat codes are still prevalent in games and QA still use them for testing purposes, but cheat codes have since become things that people expect in their games and some games aren't considered complete if they don't have them. 

But in the 1980's, one particular code stood out above all the others. In fact it was so common that it literally defined a particular company. That company was Konami. And that code is infamously known as: 

The Konami Code. 

Specifically, this code used the controller to input the following sequence: Up, Up, Down, Down, Left, Right, Left, Right, B, A. In the beginning, certain Konami games made for the Nintendo Entertainment System (NES) had this code programmed in and the code did something different depending on the game. 

The infamous Konami Code

The first instance of the Konami Code was in the game Gradius. By pausing the game (pressing Start on the controller), you could input the code and activate most of the power ups in the game for your ship after you un-paused the game. If you were playing co-op with a friend, input the Konami Code while pause and then pressed the Select button, you would activate the same power ups to the second player. 

The Konami Code gave all BUT the following power ups in Graduis: Speed Up, Double, Laser.

Unfortunately, for some time, it was believed that the code included Select and Start when inputting the code when in fact pressing Select was an option and pressing Start only paused/un-paused the game. Not all games required you to pause the game in order to enter the code and not all instances of the Konami code required input devices that featured a Start button. In addition, not all games that featured the Konami code had co-op and some games didn't even have a second player in the game.

Sorry, this is NOT the true Konami code

The code was later used in the NES game Contra, which became more popular than Gradius. This  prompted most players to nickname the code the "Contra Code" or the "30 Lives Code" as it gave players an extra 30 lives (pressing Select will also give the second player 30 lives as well). 

The code became a staple in the Gradius and Contra series of games. Other Konami franchise games would follow, including Teenage Mutant Ninja Turtles, Dance Dance Revolution, and Metal Gear (Solid). Even non-Konami games featured the code to unlock features. Games like Assassin's Creed III, BioShock Infinite, Borderlands 2, Half-Life 2, even Tony Hawk's Pro Skater 2 all had the Konami code programmed into them. 

In Dance Dance Revolution: Ultramix (XBox), if a gamepad is plugged into the fourth controller port, and the code is entered during the credits, all of the game's hidden songs are unlocked instantly.

But the code moved beyond the game world. In addition to hundreds of games, the code is featured in major movies, TV shows, and websites…all as Easter Eggs! In TV and movies, they aren't entered in as codes, they are blatantly spoken out as a nod to Konami. In games, it does a variety of things from extra lives, to access to higher difficulty levels, to unlocking all available songs, to even messages saying, "There's nothing here" (which, in itself is an Easter Egg!) featured in the original Castlevania on Japan's Famicom console system (Japan's version of the NES). On some websites, the Konami code uncovers very strange imagery and sound bytes. But to name all instances of the Konami Code (and what they do) in games and media would be a task in itself…fortunately, there are websites and even Wikipedia pages dedicated to showing all instances of the Konami Code.

If you go to digg.com and enter the Konami Code you'll…well just try it :)

So the programmer's tool that became an Easter Egg has now become more of a pop culture icon. In some ways, it has transcended traditional Easter Egg boundaries and has become its own entity. It's a voice of a generation. It harkens to a time when the first generation of Nintendo players thought they were secretly hacking their favorite game. It was the secret only the privileged video game players knew.

Now it's a secret (almost) everybody knows.