I like dynamic desktop backgrounds, so I figured I should share this find: Desktop Earth 2.0.
When the original Desktop Earth came out, I used it on my laptop as a dynamic background. The new version uses higher-resolution images (via Blue Marble) and can overlay actual cloud data (a few hours old). It changes one a schedule, giving you a visual of the sun's coverage at the time.
If you don't want the dynamic background, you can try NASA's Blue Marble web site, or have Desktop Earth Online generate a static background for you.