Android: It's a safe bet that you see your Android phone's wallpaper more often than you see your desktop's wallpaper. With Wallpaper Changer, you can set it to rotate periodically among images that you select.Wallpaper Changer isn't the only app in the Android Market that does this, but it is one of the few that's specifically optimised for performance and battery life. It runs at the interval you set, switches your wallpaper to another image in your gallery, and relinquishes the memory used to the system. The app also has a home screen widget you can use to switch to the next wallpaper in the rotation with a single tap.
It would be nice if you could add a folder of images to Wallpaper Changer instead of adding them from the Gallery app, but considering how lightweight it is, it's difficult to complain. Wallpaper Changer is free in the Android Market.