PWM is a rather lightweight WindowManager. It is remarkable in that it was the first to implement tabbing. Tuomo Valkonen wrote it to test this concept, which means allowing multiple client windows to be attached to the same frame so that large numbers of windows, particularly the hordes of terminal emulators, can be better organized. FluxBox, Pekwm and many others subsequently borrowed this feature.
Tuomo found it didn't suffice as a means of increasing the efficiency of use of GUI applications and has since moved on to write Ion which is actively developed, while PWM maintenance is on the back burner.
If you do not want to abandon overlapping windows and can live with very plain looks, this is still a WindowManager to check out.