Konami Code is the famous cheat code used in Contra, the old-school Nintendo NES game. To perform the cheat, the following keys are to be pressed.
Up, Up, Down, Down, Left, Right, Left, Right, b, a
There have been a number of websites that use Konami Code to perform easter egg, including Facebook, Google Reader, Digg and jQuery. For the full list, it is no better than to visit konamicodesites.com.
For the developers who wants to implement interesting easter eggs, or got bored with boring coding, you are encouraged to read this.