How much I love windowmaker...
Now it is much less used than once... Most people do use KDE or GNOME (or XFCE) and the good old Window Managers are kind of forgotten (or maybe simply unknown to the masses). However, when I started KDE was slow and took every single byte of RAM from my machine. GNOME was somewhat lighter. However, the desktop environment was not polished and practical (it wasn't even the 2.0).
In fact some window managers/desktop environment had higher usability (at least in my opinion) even though you had to renounce to a good file-manager (and back than gmc/nautilus was not as good as today).One of those is WindowMaker.
Simple and powerful. As it is today. Well... I restored it on my Ubuntu. Without the debian menu you have to build the application menu yourself (and that is a pain in the ass). However, since most of the times I'm only using firefox, emacs and a terminal, I managed to do it in a very short time.