PWM, "the first tabbed window manager." Multiple windows can occupy the same space, with tabs in the title bar allowing access to each. The foreground has two xterms connected in this manner. Theming consists of changing the colour scheme and display fonts.
I used version 1.0 from July 2000. There's been very little change in the WM since then, only maintenance (the author is mostly working on Ion), but there is a version 20030617 available.
Man page is good and extensive, but some crucial details are missing and I had to go look at the FAQ on the webpage to get a few pointers.
The author supplies what he calls "themes," but they change only the colours and display fonts, nothing else.
One of the first things you notice about PWM is the utter lack of buttons on the titlebar. One of the next things you notice as you adjust to the WM is that you don't miss them much. But you will become familiar with your keyboard shortcuts to use this one properly.
By default PWM uses SloppyFocus, and has four workspaces (accessible with Alt-number). These things are easily changed in the configs, which are a little easier to understand than most. Creating new keybindings (mouse as well as keyboard) is very simple, as is editing the menu. The menu is accessed by right-clicking on the desktop, and it can be made sticky.
PWM's claim to fame is its ability to let more than one window occupy the same space while remaining accessible through tabs. Attaching two windows together is done by middle-clicking and dragging a window's tab onto another window's tab. Once the two are joined like that, you can access the dormant windows by middle-clicking on the tab - I set Alt-P and Alt-N keyboard aliases to cycle through tabsets.
The author is now working on Ion, which he considers a more appropriately keyboard-driven WM, but I was less happy with. I found this WM an excellent compromise between keyboard-driven and GUI behaviour.
Extremely small. Excellent keyboard support. Easy to edit configs.
Incomplete documentation. Limited theming.