Google logos

One day I've discovered that Google sometimes changes its own logo.
Here you can see, which logos could have been seen so far.

ars gratia artis	These logos are imitating the style of famous artists.
science
sport	Every big sport event comes along with some new logos
Dilbert	A small episode of one of the world's most famous office workers.
holidays	It's holiday, it's time to change the logo.
special days
logos	Best of the rest.
operating systems	Show your sympathy with your favorite OS.
international	Search for everything you want in your preferred language.