John's Background Switcher - Switch backgrounds to trending imgur, reddit, tumblr threads
Backdrops - The only wallpapers you'll ever need. Say hello to Backdrops.
Variety Wallpaper Changer - Variety is a wallpaper changer for Ubuntu which is featureful, yet slim and very easy to use.
Peppy Wallpapers - New free wall app with some lovely minimalist designs.
Muzei - Muzei is a live wallpaper that gently refreshes your home screen each day with famous works of art.
WallpapersCraft - HD & 4K wallpapers