KDE is a powerful Open Source graphical desktop environment for Unix workstations. It combines ease of use, contemporary functionality, and outstanding graphical design with the technological superiority of the Unix operating system.
Find it at KDE Home
Components of KDE are largely licensed under the LGPL, and many of them represent libraries written in C++. It uses the popular Qt graphics library. Where the competing GNOME project tries to be fairly language-neutral, with library bindings available in many languages KDE is largely biased towards C++.