CTWM is an extension to twm, originally written by Claude Lecommandeur
that support multiple virtual screens, and a lot of other goodies.

You can use and manage up to 32 virtual screens called workspaces.
You swap from one workspace to another by clicking on a button in
an optional panel of buttons (the workspace manager) or by invoking
a function.

You can custom each workspace by choosing different colors, names
and pixmaps for the buttons and background root windows.

Its main features are:

 - Optional 3D window titles and border (ala Motif).
 - Shaped, colored icons.
 - Multiple icons for clients based on the icon name.
 - Windows can belong to several workspaces.
 - A map of your workspaces to move quickly windows between
   different workspaces.
 - Animations: icons, root backgrounds and buttons can be animated.
 - Pinnable and sticky menus.