Ranwall is an Open Source wallpaper app providing random images pulled from pre-selected websites that can easily be set as wallpaper. It is a freeware random wallpaper application that's well-designed.
Here are some screenshots to illustrate the features and user interface of this application:
Bing Wallpaper
Lively Wallpaper
DisplayFusion
John's Background Switcher
Kiwi Random Wallpaper