Openbox
Openbox is a free window manager that works on X Window System platforms, licenced under the GNU GPL. Openbox was originally derived from Blackbox 0.65.0, but has been totally rewritten in C and is from version 3.0 not based upon any code from Blackbox, which is written in C++.
Openbox is, like Blackbox, a very minimalistic, fast and extensible window manager.