Variety Wallpaper Changer - Variety is a wallpaper changer for Ubuntu which is featureful, yet slim and very easy to use.
Wallcat - Enjoy a new, beautiful wallpaper, every day.
John's Background Switcher - Switch backgrounds to trending imgur, reddit, tumblr threads
Splashify - Beautiful desktop wallpapers for Mac and Windows
Wallpaper Engine - Wallpaper Engine enables you to use live wallpapers on your Windows desktop.
Irvue - (previously Unsplash Wallpaper) Gorgeous wallpapers on your Mac.