Mac: Kuvva is a free app for Mac that automatically downloads and displays new wallpapers on a schedule you set. Whether you want a new wallpaper every hour, every day or every week, the app can handle it.
OS X can already rotate wallpapers on a regular basis, but you have to have the images on your system for it to work. Kuvva has its own collection of high-resolution images, and downloads new ones as needed (the old ones aren't kept around wasting space. The app can be set to change your background every week, every day, every 12 hours, every hour or manually. The wallpapers used are all sized for your system's native resolution, and each time it changes, you'll get a brief notification with the name of the artist and a little information about what you're seeing.
Kuvva has three image "streams" to choose from: the weekly featured artist; popular images; and your personal favourites. If you sign up for a free Kuvva account, you can build your own collection of favourites from all of the wallpapers available. If you prefer to let Kuvva handle the heavy lifting, tell it to pick from either the popular or "featured artist" feeds, and the app will do the rest.
If you like customising your desktop or enjoy changing your wallpapers, but don't like downloading them all the time, Kuvva is worth a download. It's free, and available in the Mac App Store. If you have an iPhone, the developers also have a similar app available for the iPhone.