Fvwm as a window manager construction kit
I've mentioned in the past that I use fvwm as my window manager. However, that doesn't tell you as much as you might think about what my environment looks like because fvwm is not so much a window manager as a window manager construction kit, in much the same way that exim is not a mailer but a mailer construction kit.
Historically (and currently) there have been (at least) two sorts of window managers for X. The first sort is more or less fixed window managers such as Metacity, where most of their behavior and look is hardcoded and can at most be customized somewhat through various settings and options. The other is what I'm calling window manager construction kits, where you don't so much customize them as create a complete configuration and where much of the window manager's behavior is changeable.
(Note that both sorts of window manager can have a good 'out of the box' experience, without you configuring things. The difference is that with a window manager construction kit this happens because someone wrote a good default configuration file; with a window manager, the experience is just how the window manager works.)
Programmers like window manager construction kits for all of the obvious reasons; it's like coding and we generally have a burning urge to customize our environment. Normal people generally hate them, partly because very few normal people want to choose options and customizations by learning a new language and a complex environment and then editing a file that can totally destroy your X session if you get things wrong.
I like fvwm and I'm pleased that it's finally officially released a stable version (2.6 was 'in development' for years and the 2.5.x development series long since converged to stability). But I don't fool myself that this news is of any general interest.
PS: of course there are intermediate stages between being a fully fixed window manager and being a pile of construction pieces. There are any number of window managers that have fixed ideas about some window management issues but have you construct everything else; I believe that some number of tiling window managers fall into this category.