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.
FluxBat is an ACPI based battery monitor for the fluxbox window manager. It includes two parts: a daemon process that runs the update script and the update script itself. The whole software contains only about 100 lines of code and is easy to use by including only one line in the menu configuration file of fluxbox.
i3 is a dynamic tiling window manager. Its key features are correct implementation of Xinerama (workspaces are assigned to virtual screens, and it does the right thing when attaching new monitors) and XrandR support (which is still unfinished). Both horizontal and vertical columns can be used in tiling. There is a special focus is on writing clean, readable, and well documented code. i3 uses xcb for asynchronous communication with X11, and has several measures to be very fast. i3 is primarily targeted at advanced users and developers.
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.
FBUI is a small, fast in-kernel GUI windowing system for Linux that supports overlapping windows and windows on every virtual console. Its core is currently about 50kB large. See "libfbui" for sample programs and API. This project, while fun and challenging to develop, is now defunct in recognition of the security problems that putting a windowing system inside the kernel gives rise to. The code base remains available, however, in case parts of it may prove useful to the public.
sawfish.wm.ext.pager is a C/Lisp extension that provides a functional desktop pager for the sawfish window manager. A pager is a map of your desktop. It shows not only the visible part of your desktop (the current viewport), but also the parts that extend beyond the sides of your screen. Also, if you have more than one workspace, the pager will follow you to where you are, or optionally show all workspaces at once. Of course you can select viewports and windows, and also move or raise/lower the latter.