I saw a couple of National Geographic wallpapers and those are pretty nice. I did not find an API on their site to download those on a regular basis, but it seemd pretty simple to just download them.
I just uploaded my code as a gist per https://gist.github.com/3002699 (the gist code should be more up to date than the following code)
And I added the script in my crontab:
Let me know if you have any notes/comments/changes
Here’s the code