TWM

From Grokdoc

TWM is possibly the most basic window manager, its features include window borders, a root menu, an icon manager, and an easy to edit configuration file.

Although nobody knows what it really stands for, it is most likely Tom's or The Window Manager.

If you want something that doesn't get in your way and uses almost no resources, this one is probably right for you.

It's as ubiquitous as vi since by default it is packaged with the X11 Window System.