E17 stands for Enlightenment 0.17, which is window manager for X-windows.
Since 0.17 all code has been rewritten and Enlightenment has got a more modular approach, making use of parts like evas, ecore, ebits, ewd, ewl, etox and edb - which all except ecore, could be considered separate programs, in addition to being components of the Enlightenment window manager.