I've been using Window Maker as a secondary Window Manager for years. In fact, I spend much of my work life interacting with it, because I use VNC sessions extensively, and use Window Maker inside VNC. Not only is it lightweight (not having all the bloat of a full-blown desktop environment) but it can be made to look very plain, which is ideal for VNC. (And having a different environment inside VNC means I can customize my desktop independently without worrying about some of it accidentally interfering with my VNC sessions. In particular, my main desktop has quite a bit of eye candy.)
Solaris 10 doesn't come with Window Maker by default, although an old version is on the Companion CD (installed in /opt/sfw). I usually use my own version, which is up to date.
As a convenience, I've made my build available as a Solaris package. Choose the right file from the following list:
To install,
pkgadd -d PCTwmaker.i386.pkg PCTwmaker
where you should replace "PCTwmaker.i386.pkg" with whichever file you downloaded.
(Using pkgadd should work on OpenSolaris distributions too. You can safely ignore any conflicts marked as "attribute change only". And binary compatibility should ensure that the software works correctly as well.)
One of the great features of Window Maker was the availability of a large number of themes. Sadly these have largely vanished from sight. I have, however, created OpenSolaris and OpenIndiana themes and included them. If you've already got Window Maker installed and just want the themes, here they are: OpenSolaris, OpenIndiana.