Subversion is used as the version control system throughout KDE. CMake is also used, and KDE played a heavy role in its development. Finally, KDE also served as a sufficiently complex target for Valgrind.
MediaWiki is used for user-editable websites, although KDE's own Capacity is also being used.
Several libraries from freedesktop.org are used in KDE to run the basic desktop and window manager.
The GGZ Gaming Zone project develops specifications for free online gaming. KDE contains some libkggz* libraries to interface with GGZ components. Learn more at the KGGZ architecture page.