Fluxbox is a super-wicked window manager for the X Windows System. It is based on the much-loved blackbox, known for small footprint, themeability, and minimalist design. Fluxbox expands on blackbox by adding some key features while still maintaining the lightness that blackbox is liked for.

Some of the features fluxbox adds are "out-of-the-box" support for hotkeys, "tabs" on every window that let you consolidate windows in the same space, and expanded styles. At the same time, it posesses all blackbox's features, including virtual desktops, icon support, and the mystical "slit."

More info at http://fluxbox.sourceforge.net.