The GNOME desktop environment - https://www.usenix.org/legacy/publications/library/proceedin...

"Sadly, the KDE effort is based on the Qt user interface toolkit which is a non-free library. The main problem is that modification of the toolkit is not allowed. The ability to modify the interface toolkit is a requirement of free software[6] and of the Open Source Software Guidelines[17]. Although Qt is released in source form, permission to redistribute changes made to it is denied."