National Geographic has the best feature, Photo of the Day. I added the widget to my homepage so every day I could see the newest picture, and any that I was most interested in became my desktop photo for somewhat long periods of time. These puppies are pretty old, but I keep coming back to them whenever I need to refresh my screen...
1. Apricot Blossoms
2. Crested Crane
3. Oregon Sand Dunes
4. Canary Islands Surf Beach
5. Young French Polynesian Turtle