Sawfish is a GNOME compliant Window Manager for X11 (aka X, X Windows). It used to be called Sawmill but had to change its name due to a conflict with a commerical log analyser produced by Flowerfire (there's also an IBM operating system project with the same name).
Sawfish's philosophy is simplicity, extensibility and flexibility. Somehow, it achieves this by using the Lisp programming language as a framework (this is quite neat, but if you don't grok Lisp, theme-authoring is virtually impossible). All its functions and features can be modified by means of a custom .sawfishrc Lisp script, and all the high level window manager functions are implemented in Lisp to allow for future development.
Sawfish is very mature
and comes with a couple of very good config
utilities - there's the GNOME Control Centre
Applet and a standalone application called sawfish-ui
. They let you tweak just about everything you can think of, and work very well.
Sawfish was adopted as the default window manager for GNOME in late 2000, replacing Enlightenment, and it's part of the Ximian GNOME distribution (although now the "default" window manager for GNOME is Metacity, a window manager that draws a lot from Sawfish, but is much more closely tied to GNOME).
Sawfish can be themed like most X11 window managers, there's even a plugin for the GIMP called GimpMill that can be used to create themes directly from graphics, which is cool. But you still need to know Lisp, which ain't. However if you're lazy like me, you can download hundreds of Sawfish themes (by people like tigert) from themes.org.
Sawfish's homepage is at http://sawmill.sf.net/.