Linux: Slidewall is a free utility for Linux systems that lets you change your desktop background at a specified interval. It can pull images from a folder on your computer or popular wallpaper site Wallbase.
Slidewall was designed for Ubuntu's Unity UI, but it works just fine in other versions of Linux, as long as you're using Gnome. Once installed, drag over the images you want Slidewall to choose from, tell it how frequently you want the wallpaper switched, and sit back and let the app do its job. If you prefer to be surprised, you can tell Slidewall to download new or random wallpapers that have been posted to Wallbase instead. Wallbase also has a "Live Wallpaper" option that superimposes a map of the world showing night and day zones on your desktop.
Ever since I installed Linux on my old Macbook Pro I've been looking for a good wallpaper manager, and Slidewall looks like a great option.