LXDE

LXDE is a desktop environment for Linux and Unix-like operating systems. It uses Openbox as its default window manager, but can be configured to use other window managers.

Distributions that provide it as default GUI
Here provides a list of what distributions use it as a default/alternative desktop environment out of the box.
 * Debian
 * Fedora
 * Linux Mint (formerly)
 * Lubuntu (formerly)