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The KDE 4 Desktop has gained some major new capabilities. The Plasma desktop shell offers a new desktop interface, including panel, menu and widgets on the desktop as well as a dashboard function. KWin, the KDE Window manager, now supports advanced graphical effects to ease interaction with your windows.
Lots of KDE Applications have seen improvements as well. Visual updates through vector-based artwork, changes in the underlying libraries, user interface enhancements, new features, even new applications -- you name it, KDE 4.0 has it. Okular, the new document viewer and Dolphin, the new file manager are only two applications that leverage KDE 4.0's new technologies.