ZWM is an SDL-based window manager and widget library intended to be used in cross-platform applications. It features basic widgets like buttons, lists, containers, a file browser, combo-box, and many more. It also features support for mutimedia playback (MPEG, MP3, WAV, MIDI, Ogg, etc.).
spectrwm is a minimalistic tiling window manager that tries to stay out of the way so that valuable screen real estate can be used for much more important stuff. It has sane defaults and does not require one to learn a language to do any configuration. It was written by hackers for hackers and it strives to be small, compact, and fast.
Stem Desktop is a minimalistic Debian desktop for low-end hardware with limited memory (166Mhz / less than 64M). The goal is to find programs that have the smallest possible memory footprint. After installing *.deb packages on top of clean Debian, the desktop is ready to accept graphical logins.
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-Crystal aims to create an easy to use, pretty, but also powerful desktop environment for Linux or other Unix-like operating systems. It uses the following programs: FVWM as a window manager and "main core"; different ways to provide icons on the desktop including support for many file managers and custom commands; xterm, aterm, mrxvt, or urxvt as terminal emulators; MPD or XMMS2 as music players (there's built-in support for controlling these programs), and several other tools for different functions, like setting a wallpaper or taking screenshots.