Windows only: If you're a fan of the high quality photographs regularly rotating on the Bing homepage, free application ZapBing brings a rotating lineup of those images to your desktop wallpaper.
(Click the image above for a closer look.)
The site's in French, so I've linked to both the original version and the translated English version below. One note, though: You'll need to visit the original version to download the app (Google Translate borks the download link). Find and click the orange download button pictured at left to get the app. Once it's installed, you can change the app's default language to English by right-clicking the system tray.
Once you're all set up for English, you'll need to make a few tweaks to get started, including ticking the Display as wallpaper box. You'll also need to set the Mode, choosing among Polaroid (pictured above), centered and fullscreen. Once you've picked all your preferred settings, click the red and blue rotating arrows button and ZapBing will download the image and set it as your wallpaper. (If you've chosen Polaroid mode, it'll also theme the picture.) If you don't like what it's come up with, just double-click the system tray icon to load a new image.
ZapBing is a freeware, Windows only.