Muzei - Muzei is a live wallpaper that gently refreshes your home screen each day with famous works of art.
Becrux Wally - Wally is a Qt4 wallpaper changer, using multiple sources like files, folders, FTP remote folders, Flickr, Panoramio, Ipernity, Photobucket, Buzznet, Picasa, Smugmug, Google, Vladstudio and deviantART images. Now it's available in many languages!!
Variety Wallpaper Changer - Variety is a wallpaper changer for Ubuntu which is featureful, yet slim and very easy to use.
AutoWallpaper - Using AutoWallpaper:
John's Background Switcher - Switch backgrounds to trending imgur, reddit, tumblr threads
Kuvva - Kuvva is the source for beautiful wallpapers.