Note: This window manager was originally known as Sawmill, but was changed after it was found to clash with an existing product.
Sawfish is an extensible window manager using an Emacs Lisp-like scripting language. All window decorations are configurable and all user-interface policy is controlled through the Lisp language. But Sawfish also comes with a GUI configurator which allows basic customization (including switching themes) without having to know any Lisp.
Sawfish focuses mainly on its core ability to manage windows, and not on all the extras like application docks and modules that many modern window managers provide. This means that Sawfish works really well when used in conjunction with a desktop environment, particularly GNOME, as that provides just the facilities that Sawfish lacks. In fact, Sawfish was once the default window manager for the GNOME desktop.
For further information and downloads, see the Official Sawfish site.
See also the Sawfish themes at Freshmeat page.