As a web application starts to scale to higher levels, you need to be able to quickly isolate the sources of slowness... even if it's just a clue to get you started on your way to finding the problem area.
For that reason, my favorite “easter egg” happens when you mouse over the copyright symbol in the footer of one of my sites.
As an example, mouse over the "©" symbol in the footer of this page:
Without looking below at the comments on this post, can you guess what that number represents? Hint: I borrowed this idea from Facebook when I saw it on their footer three years ago.