Enlightenment is a completely themeable, highly configurable WindowManager for X11. It is currently up to release 0.16, commonly refered to as E16, while a major rewrite is underway (and has been underway for years) for Enlightenment 0.17 (E17) which is planned to become a full DesktopEnvironment.
That's the short version of the story, anyway. DR17 was almost released at some point, but Raster 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.
See also: LookingGlass
Part of CategoryWindowManager
2 pages link to Enlightenment:
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