Golem is a window manager for the X Window System, developed by Jordan DeLong. The author develops and maintains the project from a FreeBSD box, although Golem also happily runs on Linux. It has only been tested on the x86 platform, but judging from its source code, it doesn't appear to have any platform-dependent nastiness in it.
In fact, it doesn't really have much nastiness in it at all, because it doesn't have much anything in it. It just manages windows. It has a plugin-based architecture that allows it to be configured to support various things, like a taskbar, a pier (like Window Maker's dock), Enlightenment-style animated window open and close operations, and such stuff. It can also be configured to not have any of all that, if you prefer to have other programs handling those things, or just don't want them. Configuration is done in the old-school UNIX way, by editing a configuration file (typically in ~/.golem/golemrc) with a text editor.
While it can be used with GNOME or KDE, it is also very well-suited for people looking to put together a homebrew desktop environment. If you like the UNIX paradigm of small components that do one thing, and do it well, you need to comment out four lines in the aforementioned file to have a window manager that only manages windows and nothing else.
Golem is quite themable, and its theme format is not quite as messy and arcane as the one found in Enlightenment. It also doesn't require you to learn LISP, like Sawfish does. Unlike those two, a Golem theme must store its graphics in the XPM format. The jury's still out on whether that's a feature or a bug, though.
The current version is 0.0.5, and was released on the 21st May, 2002. Although the version number pretty much screams "beta version" right into your face, it appears to be a quite stable and usable program. The project has a website at http://golem.sourceforge.net/ .
If you like your window manager to be big, featureful and manage things not related to windows (like putting graphics on your root window, or popping up tooltips and help), Golem probably isn't your best choice. Go with Afterstep, Sawfish or Enlightenment instead. If you want to be able to reconfigure your window manager using a GUI, Golem will also sorely disappoint you. Enlightenment is hard to beat in this area anyway, unless you use GNOME or KDE, in which case Sawfish, Metacity and KWin are even better.
If, however, you just want your window manager to manage windows without being too minimalist, Golem is definitely a nice option. Similar programs include Sawfish, Oroborus and Phluid.
As an aside, the binary size of Golem (compiled on my i686 box, with the -O2 compiler option set) is just 83 kilobytes. Oroborus is even smaller at 42K, Sawfish takes up 154K, and Enlightenment takes up 665K.