Great images for your desktop are great on their own, but wallpaper software can make them even better. Here are a few apps that will allow you to keep your wallpapers fresh, add video and effects, and a lot more.
Mac: Satellite Eyes changes your Mac's desktop image to a satellite view of your current location. It uses Wi-Fi to figure out approximately where you are, grabs satellite images of that location, and then sets them as your wallpaper. Obviously, it's not an exciting prospect for desktop machines, but those of you with MacBooks will appreciate the novelty.
Download Satellite Eyes (Free)
Windows: Living Earth puts NASA satellite images of the entire earth on your desktop. It also provides weather. In addition to providing a live desktop in Windows, it also doubles as a screensaver.
Windows/Mac/iOS/Ubuntu/KDE4: Vlad Studios offers a number of Wallpaper Clocks, which are -- believe it or not -- wallpapers with built-in clocks. They're not apps or simple image files, but rather WCZ files that can be used by a number of wallpaper clock apps. If you check out Vlad Studio's page, you'll see a list of apps for several platforms on the left side of the page.
Download Wallpaper Clocks (Free) [Vlad Studios]
Windows: DeskScapes provides you with a collection of hundreds of animated wallpapers. You can also add animated effects to existing wallpapers. This can involve adding birds in the sky of a photo you love or stylising any image in various ways.
Download DeskScapes ($US20, free trial)
Mac: Desktop Aquarium's name should imply what it does: put an aquarium on your Mac's desktop. The free version includes shorter videos (30 seconds) and occasional popup ads. If you want longer videos and no ads, you can get the full version for $2.99.
Desktop Weather and YWeather
Windows: Always want to know the weather? Desktop Weather will put it on your desktop. In addition to an actual image, it displays a forecast as well.
Download Desktop Weather (free) [Wallpaper Software]
Mac: YWeather is a Perl script that does the same sort of thing on OS X using the popular desktop customisation app GeekTool.
Download and Setup YWeather (free) [Lifehacker]
Desktop Wallpaper Construction Kit
Mac: Want shelves on your desktop? This app will help you take a wallpaper you already like and add them. You can choose the number and types of shelves you have, plus add a floor for those shelves to sit on.
Download Desktop Wallpaper Construction Kit ($0.99) [iTunes App Store]
Windows:NatGeo Downloader can grab over 5000 wallpaper-worthy images from National Geographic's library. If your desktop is generally sporting a rotation of nature and wildlife photos, you'll want to grab this app.
Windows: If you don't want to bother looking for wallpapers but want great ones cycling through your desktop, Pulse will download and display them automatically based on any keyword of your choice.
Download Pulse (free)
Add Movie Wallpaper
Mac/Windows: Using the app Add Movie Wallpaper, you can add video to your Mac's desktop. Windows users can do this for free with VLC.
Download Add Movie Wallpaper ($1.99) [iTunes App Store]