John's Background Switcher - Switch backgrounds to trending imgur, reddit, tumblr threads
Variety Wallpaper Changer - Variety is a wallpaper changer for Ubuntu which is featureful, yet slim and very easy to use.
WallpapersEveryday - Gorgeous wallpapers on your desktop in just one click or auto-delivered at chosen time intervals.
Wallpaper Cycler - NuonSoft WallpaperCycler is a powerfull utility to manage all your wallpapers and to automatically cycle between your wallpapers.
Artpip - Artistic wallpaper for your desktop, refreshed daily.
Muzei - Muzei is a live wallpaper that gently refreshes your home screen each day with famous works of art.