The Harmony toolkit is a never-completed free software widget toolkit that aimed to be API compatible with the Qt widget toolkit. The QPL license that Qt used was proprietary, merely providing the toolkit free of charge if the program was not sold for profit and if its source code was freely available.
The GNU project launched the Harmony project, and also the GNOME desktop project, to counter the problem that the free software KDE desktop was gaining popularity, but requiring that people install proprietary software.
In addition to source compatibility with Qt, the Harmony project also aimed to add functionality such as multi-threaded applications and pluggable themes; features that Qt itself later added.
Development ceased at the end of 2000, when Qt was released under the GPL.
In January 2009, Qt was made available under the GNU LGPL, along with the previous license options.