An Easter Egg is something a developer puts into his software for fun, there is tons out there, in all big softwares, it's hidden features (there is a space invaders game in Excel 7 for instance!)
There is one on NOOL, I don't remember a lot of people finding it and telling us about it, so I'll reveal it a little.. if you click the color code at the top right of your screen, it will reveal it.
It's good to see it once in a while ;)
P/S: I didn't do it! It's Mr Greenwood (Boisvert) and Vincent Dvorak!