WindowMaker is an often forgotten lightweight window manager. Also I tend to forget it when using a modern desktop, but when I have to use an old computer I sooner or later come back to WindowMaker.
Now that I'm using my 1 GHz 'Oikos' desktop I've again installed WindowMaker. It's design is very different from most of the other window managers and desktops, as it does not have a panel or a start menu for starting applications. Right-clicking the root window does, however, open a context menu for running applications, opening XTerm, changing the visual appearance of WindowMaker and so on. More functionality can be added by installing and running WindowMaker applets.
1) Default look
WindowMaker's default desktop is probably not the prettiest of all desktops. There are, however, hundreds of different themes that can be used for modifying the simple desktop to become simple, strong and sharp. Freshmeat has an excellent collection of themes for WindowMaker.
2) Aay: A light grey theme
5) PureWM is another light theme.
These light themes are the most usable ones with my old recycled 17" monitor. Your hardware might like some other combinations of colors better, remember to experiment also with themes that are not provided by your distribution. Many more beautiful themes and icon sets can be found in Freshmeat!
Installing a theme is not difficult. They are distributed as .tar.gz packages. Gunzip and tar xvf them in the directory ~/GNUstep/Library/WindowMaker/Themes and you will find the just installed theme by right-clicking the root window (wallpaper), and selecting Appearance, Themes, Theme.