Two that I remember (Score 1)

I worked for a rail company. For an intranet site I made the pull-down menu was a stylised train, with each 'carriage' being the menu option the use would click-on to trigger the pull-down. But, being into javascript as I was, I added an Easter egg such that, if the user clicked on the cab of the engine the train would start-up and accelerate off the edit of the screen, only to return with an added carriage proving a menu of company relevant specific web-links.
On a customer complaint system I developed the icon on the 'Query' button of the main screen would be replaced by an icon of a fish on 1st April. It might still be doing that for all I know....