There's another one. Go to Google.com, type in "do a barrel roll", and that's it. Note, you may need Google Chrome to make this work.
There are more easter eggs like this, I'm sure. But think of the possibilities. These may seem like novelties at face value, but I can see huge potential in them. Can you imagine how much more revenue if your site was more interactive like this? Obviously, it depends on your target market's interests.
If you had to make your website like a videogame, in order to increase visitor interactivity and profits, what would you do? How would you do it? What ideas would you try out?