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.
Athene is a desktop system for Linux that uses its own graphics drivers for high speed access to your graphics card. Because the Athene desktop and its applications have direct access to the graphics hardware, they have a significant speed advantage over conventional X applications. Athene is also backwards compatible with your existing X11 software. It features a highly responsive interface, scalable icons, translucency, alpha blending, Unicode support, and Windows compatibility.
Beast is a powerful music composition and modular synthesis application. It offers multiple input methods like multitrack, piano roll, and pattern editing and supports skins. On the technical side, it has a wide range of abilities like sequencing, unlimited undo/redo history, real-time synthesis with multiprocessor support, full duplex 32-bit audio rendering, precise timing down to sample granularity, on demand loading of partial wave files, on the fly decoding of various sample formats, aliasing free oscillators, and full Scheme scripting support.
Belt is a foldable menubar that works somewhat like the "belt" found on the Mac. It reads a simple configuration files, uses XPMs for the buttons, and has a lot of options. While not as advanced as the similar applications found in GNOME or KDE, it is rather lightweight and intended primarily for use with windowmanagers that don't have easy dynamic configuration options, like wm2 for example.
Fresco (formerly Berlin) is an experimental windowing system which is a logical extension of the integrated layout and structured graphics model developed in InterViews and the original Fresco project. It makes heavy use of CORBA for transparent inter-process control, and utilizes a loadable module framework to maintain good drawing speed and extensive customizabillity.
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.