I use Window Maker. It reproduces the elegant look and feel of the NeXTSTEP user interface. It is fast, easy to configure, and easy to use. You can find many dock applets @DockApps.org. It comes with a powerful GUI configuration editor, called WPrefs, which removes the need to edit text-based config files by hand. There are plenty of window managers for X, some of them are listed on this page. Window Maker is available for every Linux distribution, but you can always download source code and compile it.