From 1998 onwards Google has been regularly changing the Google logo up, with the new iterations labeled Google Doodles. These doodles have been used to great effect to mark significant events. They have also become a lot more involved over the years, with interactive elements being added.
Take a look the best playable Google Doodles released to date. Google doesn't delete these games, so they remain playable long after they appear on the Google homepage. Which is great for all of us who spend far too long on the Web and who are in constant need of things to occupy our brains.