PLWM (The Pointless Window Manager) is a highly modularised window manager written in Python. It has no configuration files; instead, you combine Python classes to make the perfect window manager for yourself. This is not a window manager for non-programmers. The feature list includes point-to-focus and sloppy-focus, outline move, resizing, deiconifying, views (extremely powerful workspaces), and multihead support. PLWM is controlled completely from the keyboard.
Ratpoison is a simple window manager with no large library dependencies, no fancy graphics, no window decorations, and no rodent dependence. It is largely modeled after GNU Screen, which has done wonders in the virtual terminal market. All interaction with the window manager is done through keystrokes. ratpoison has a prefix map to minimize the key clobbering that cripples EMACS and other quality pieces of software. All windows are maximized and kept maximized to avoid wasting precious screen space.
Sawfish (the window manager formerly known as Sawmill) is an extensible window manager using a Lisp-based extension language. All window decorations are configurable and all user-interface policy is controlled through the language. Its aim is to manage windows in the most flexible and attractive manner possible. As such it does not implement desktop backgrounds, application docks, or other things that may be achieved through separate applications. Sawfish is mostly GNOME-compliant; almost all configuration may be made through a graphical interface.
Scwm is the Scheme Constraints Window Manager, a highly dynamic and extensible window manager for the X Window System. Scwm embeds Guile Scheme as the configuration and extension language, and includes a sophisticated constraint solver for permitting the user to interactively specify constraints among top-level window sizes and positions. Nearly all decorations can be changed at runtime on a per-window basis. Dynamic loading of C modules is supported. Scwm is self-documenting and provides a powerful protocol for interacting with the window manager from other processes.
selectwm is a simple but robust program that will let you pick a window manager (or other executable) to run at X startup, and optionally after a window manager exits. It uses the GTK+ toolkit, and includes options like a timer to start the default window manager and modification of the window manager list from within selectwm.