Wallhaven - Your source for the best high quality wallpapers on the Net!
John's Background Switcher - Switch backgrounds to trending imgur, reddit, tumblr threads
InterfaceLIFT - High-resolution widescreen wallpapers, desktop icons, and themes for Mac OS X, Microsoft Windows...
Muzei - Muzei is a live wallpaper that gently refreshes your home screen each day with famous works of art.
WallpaperFusion - The best multi-monitor and Eyefinity wallpaper images, all in one place!
Variety Wallpaper Changer - Variety is a wallpaper changer for Ubuntu which is featureful, yet slim and very easy to use.