I don't know about you, but I live for the Google themes. A little fox playing a guitar, picking oranges and hanging up the laundry just makes my day. But now, there are a whole bunch of new themes! I currently have a family raccoons raking up leaves. I can't wait to see what they do next.
Of course, the first thing I do is look for the easter egg themes (at 3:14am something exciting happens, although I don't stay up until 3:14 waiting, I just google the XML for the raccoons, then find the style sheet for 3:14am and then find the image file in the style sheet. Way more than you ever needed to know).
Here is the easter egg I found for my little raccoons:

If you want to see easter eggs for all of the themes, check out ruscoe.net, he tracks them all.