Enlightenment is a completely themeable, highly configurable WindowManager for X11. It is currently up to release 0.16, commonly refered to as E16. The latest stable version is e16.7, and 16.8 is under development.
In the meantime, a complete rewrite is underway (and has been underway for years) for Enlightenment 0.17 (E17) which is planned to become what the the Enlightenment Development team terms a "Desktop Shell", incorporating elements of both Window and File Manager functionality, but not going as far as a full DesktopEnvironment.
That's the short version of the story, anyway. DR17 was almost released at some point, but Raster (the lead Developer) thought that releasing code that would need to be rewritten in a week wasn't right. You can see some results of that decision in the DR17 Enlightenment page. I suggest watching the movies, as static pictures don't show the potential of this desktop.
While E17 is not yet ready for release, many people are already using E17 as their primary WindowManager. It is possible to obtain a latest development version from cvs or snapshots from freedesktop.org. See get-e.org for installation details, user guides, themes, bacgkrounds, screenshots, and just to keep up with the latest developments.
See also: LookingGlass