Every now and then I come back to WindowMaker when looking for a nice lightweight window manager. It is fast and efficient, and usable even on a Pentium 100 MHz with 40 MB. Well, at least I consider it usable. YMMV.
WindowMaker was designed to emulate NeXTSTEP (http://en.wikipedia.org/wiki/NeXT). As result of this, WindowMaker looks strikingly different from the more common window managers and desktops as it has no panels on top or bottom of the screen. It supports multiple desktops just like all the other modern window managers.
DockApps are WindowMaker's small applets that can be used, for example, for mounting devices or playing music, showing time and date or monitoring the system. Most distributions install some DockApps by default, and you can find even more of them in dockapps.org.
* WindowMaker in Wikipedia