Blackbox is a fast, lightweight window manager for the X Window System, with a minimum of external library dependencies. (Compare with xfce, contrast with KDE and Gnome, both of which are more full-featured but drastically more heavyweight and complex.)
Blackbox is built with C++ and contains completely original code (even though the graphics implementation is similar to that of Window Maker). Blackbox's official site is at http://blackboxwm.sourceforge.net/ .
You may also be interested in the fluxbox XWM, which forked from blackbox at 0.61.1 and claims to be more full-featured while remaining very lightweight.