AfterStep is a Window Manager for X which started by emulating the NeXTSTEP look and feel, but which has been significantly altered according to the requests of various users. Many adepts will tell you that NeXTSTEP is not only the most visually pleasant interface, but also one of the most functional and intuitive out there. AfterStep aims to incorporate the advantages of the NeXTSTEP interface, and add additional useful features.
Blackbox is yet another addition to the list of window managers for X. It is written in C++, sharing no common code with any other window manager (even though the graphics implementation is similar to that of WindowMaker). It features small code size, a fast interface with simple menus, multiple workspaces, decorated windows, built-in graphics code to render solids, gradients, and bevels on the fly, and more.
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.
Ion is a tiling (no overlapping windows) window manager that also has PWM-style tabbed frames which can contain multiple client windows. These features help to keep windows organized and to switch quickly between them. Ion was designed primarily as an efficient and unobtrusive window manager for users who prefer the keyboard.
dvtm brings the concept of tiling window management, popularized by X11-window managers like dwm, to the console. As a console window manager, it tries to make it easy to work with multiple console based programs like vim, mutt, cmus, or irssi. dvtm is intended to be used where X11 isn't available or over SSH. In conjunction with dtach, it can be seen as a lightweight alternative to GNU screen.
FVWM is a multiple large virtual desktop window manager originally derived from twm. FVWM is intended to have a small memory footprint and a rich feature set, be extremely customizable and extendable and have a high degree of Motif mwm compatibility. FVWM is highly extensible through its module interface.
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.