Enter ↑ ↑ ↓ ↓ ← → ← → B A ENTER while logged on to Facebook to experience a special treat! To be fair, this is far less interesting than the sparkly Unicorns generated by the Konami Code on the ESPN website on April 27th. The ESPN easter egg provided intense, fleeting fun, and it's still unclear whether it was actually ESPN sanctioned. It doesn't seem unreasonable to imagine that Facebook's use of the Konami Code may not be a hack, and so it may be available to enjoy for more than a few hours.
For a more complete list of sites where easter eggs are activated by the Konami Code visit konamicodesites.com.
Most just cause an image to appear on the screen like a proliferation of zombies or prancing uniforms (Adventure Quest's are particularly perky). While I have yet to test every site listed, I wish more used the Konami Code in a spirit similar to its original purpose (to provide extra lives or powerups)to provide bonus functionality or games to the site, a la the original code.
Use the Konami Code on Jquery for an unbeatable Guitar Hero minigame.