There are hundreds of Easter Eggs that developers just love to put in just for a really cool bit, but out of all the game I found having sweet Easter Eggs, especially Nintendo's. My absolute favorite is the recently revealed Bioshock Easter Egg that blew my mind. It was confirmed in April 1st, but it wasn't really a April fools joke, it was actually real.
If you really want to know what it is even though you haven't played it yet. Don't say i didn't warn you.