Bringing back the love for Contra
Not sure if ESPN.com was the first site to do this, but back on April Fools they placed a script on the homepage that granted access to hidden easter eggs by entering the code from perhaps the greatest game of all time, Contra.
Up, Up, Down, Down, Left, Right, Left, Right, B, A, B
This prank inspired a whole army of geeks to initiate the same or similar scripts on their sites, including some of the biggest sites out there such as Google, Facebook, Mint, Digg, and of course jquery.com.
While it isn't (and can't be) a complete list, you can check out some of the more well known implementations here: http://konamicodesites.com/
Of course, you'll need the secret code to gain access....