CTWM is a highly-configurable window manager based on the classic TWM window manager. It supports xpms, multiple workspaces, advanced icon managment, animated icons and backgrounds, 3D titles and borders, etc. It offers rudimentary Gnome support and is backwards-compatible with TWM.
WMFS (Window Manager From Scratch) is a lightweight and highly configurable tiling window manager for X. It can be configured with a configuration file (made with LibConfuse), supports Xft (Freetype) fonts and is compliant with the Extended Window Manager Hints (EWMH) specifications.
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.
Enlightenment is a themeable, fast, flexible, and powerful window manager that is designed to be as configurable as possible in both look and feel. In addition, Enlightenment provides an applet API that provides functionality somewhere between the GNOME applet system and Dock Apps from Afterstep/WindowMaker. The current design aim is for Enlightenment to become a desktop shell.