This page tries to document inter-project relationships that KDE has with other (independently-run) projects. The page is far from being complete.
Note: In the dependencies list, it doesn't specify an exact list of libraries and their versions. Such a page is still needed in addition!
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 GNU project develops software, some of which is used in KDE applications, and some for development. In the beginning of KDE there was some confusion around this. However, this has long been settled, KDE developers had a tee with RMS in 2003 and KDE can be used as a totally free desktop for the GNU system.
Last but not least, KDE uses the fine Qt library from Trolltech.