Tom's virtual Tab Window Manager
tvtwm is a superset of the X11R5 release of twm (Tom's Window Manager), written by Tom LaStrange. Much of the early functionality, which is described in more detail in README.old, was added by Tom LaStrange himself. Since then, tvtwm has fallen under my control, and I've added some things myself.
The major benefit of tvtwm over twm is the "Virtual Desktop". This allows you do define a substitute root window that is larger than your display area. This new virtual root window is the parent for all of your X clients. tvtwm provides a "Panner" which will let you see a scaled down representation of the whole virtual desktop. Using this panner, and keys bound to functions added to tvtwm, you can move around this desktop to have your physical display showing only part of the whole desktop. Thus, you can have sections of your desktop assigned to particular tasks, or just use it to keep from having 20 million layers of windows. :-)