National Geographic has a picture of the day on their website that I think is really great. I go to the site every day and, if I like the picture (which I usually do), I put it as my desktop background. Consequently I often have a different desktop background every day of the week.

The picture of the birds (above) is the best one I have seen so far. It's from a few days ago, but I like it so much I think it will be sticking around for a while.