Xmonad is nice, but Awesome's tags beat it IMO. You don't really have multiple desktops, you have multiple tags. Each window can have multiple tags and there can be multiple tags visible on the screen at any moment. So what you end up seeing is a union between all the windows with any of the tags that are set to be visible.

Awesome is both really minimal and really extensible. By default you don't have anything, and you build it up to how you like it with lua and a couple of libraries that expose bindings to things like desktops and tags.