I logged into Gmail this morning and noticed that things looked different. Then a message popped up telling me to check out the new Themes tab. Holy crap! That’s awesome! I’m rockin’ the Desk theme right now. It appears that some of them actually change throughout the day to reflect the weather in your location. Sooooo cool! (Note: It appears they’re rolling this out slowly. Snookums doesn’t have it on his account yet, nor does anybody else in my office.)
Edited to add: The Gmail blog has more info and pictures of the themes.