Linux: DesktopNova is a utility for Gnome or XFCE-based Linux systems (and was built for Ubuntu) that will rotate your desktop wallpaper on a schedule that you select, through a folder of images that you specify.
You can install DesktopNova via apt-get or by downloading the latest tarball from the developer's site. Once installed, open the utility to specify the folder full of images you'd like it to cycle through and how frequently you'd like the wallpaper to change. You'll need to make sure the daemon is running at all times for the app to change wallpapers, but once it's started you shouldn't have an issue.
One bonus that DesktopNova has over its built-in Mac OS counterpart and its Windows alternative, the previously mentioned John's Background Switcher, is that you can tell the app to switch wallpaper by scrolling the mouse wheel, or open it from the tray to switch wallpapers at any time.