NeoDownloader - As you know, there is a lot of websites with a lot of pictures, music and movies.
John's Background Switcher - Switch backgrounds to trending imgur, reddit, tumblr threads
Vov Picture Downloader - Download pictures from websites.
Variety Wallpaper Changer - Variety is a wallpaper changer for Ubuntu which is featureful, yet slim and very easy to use.
Persepolis Download Manager - Persepolis Download Manager is a GUI for aria2.
Muzei - Muzei is a live wallpaper that gently refreshes your home screen each day with famous works of art.