John's Background Switcher - Switch backgrounds to trending imgur, reddit, tumblr threads
Wallpaper Cycler - NuonSoft WallpaperCycler is a powerfull utility to manage all your wallpapers and to automatically cycle between your wallpapers.
Fondo - Find the most beautiful wallpapers from Unsplash.com
Variety Wallpaper Changer - Variety is a wallpaper changer for Ubuntu which is featureful, yet slim and very easy to use.
Muzei - Muzei is a live wallpaper that gently refreshes your home screen each day with famous works of art.
MuralPix Wallpaper Changer - MuralPix Wallpaper Changer and Screen Saver