KDE [http://www.kde.org] is a desktop environment that works with many platforms.  It uses the Qt toolkit by Trolltech [http://www.trolltech.com/]

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Tk_Theme [http://www.xmission.com/~georgeps/Tk_Theme/] is an extension that provides a -kdesystray option that allows you to write applications that use the KDE system tray.

 [TODO write about the Tk extension that makes Tk use the Qt colors]