aewm is a minimalist window manager for X11. It has no nifty features, but is light on resources and extremely simple in appearance. It should eventually make a good reference implementation of the ICCCM. A few separate programs are included to handle running programs, switching between windows, etc.
Phluid is a window manager that emphasizes efficiency, speed, and beauty. It uses rasterman's Imlib2 library as the image/font rendering backend. It also attempts to be flexible and compliant with the ICCCM standard and gets a lot of its inspiration from the enlightenment window manager, but some of the initial code design is based on aewm.
The Virtual Tab Window Manager (VTWM) is a virtual window manager with adjustable graphical complexity. With minimal settings, it is ideal for limited resource situations, using little memory, little CPU time, few colors, and little bandwidth. Fully blown, it supports m4 and regex processing of the resource file, sound effects, user-defined color icons and buttons, and more.
WMI is a lightweight window manager for X11 which attempts to combine the best features of larswm, ion, evilwm, and ratpoison into one window manager. It supports both the conventional way of managing windows in a overlapping manner and the screen-alike way of managing windows in a non-overlapping fashion.
wmii is the next generation of wmi. It is a highly customizable keyboard-and-mouse-driven X11 window manager that supports conventional, tabbed, and tiled window management with low memory usage and vi-like user interaction. Due to a complete rewrite, it is highly modularized and uses a new configuration and inter-process communication interface, which is oriented on the 9p protocol of the Plan9 operating system.
libDockApp is a simple library for easily writing Window Maker dock applications, also known as dockapps (those that only show up in the dock). It is very limited and can only be used for dockapps that open a single appicon for each process in only a single display, but this seems to be enough for most, if not all, dockapps.