From TIME magazine:
The Konami code, a key sequence — up, up, down, down, left, right, left, right, B, A — that, if entered, gave the player a set of the game's power-ups can be used in Facebook.
If you type it in it gives a cool lens flare effect. Try it out.
The code has also been incorprated into many other websites and games. A list can be found on a dedicated website — Konami Code Sites — , of course you have to type the code to access the site.