Harmony toolkit
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The Harmony toolkit is a free, never-completed widget toolkit that aimed to be API compatible with the (at the time, proprietary) Qt widget toolkit, but wanted to add functionality such as multi-threaded applications and pluggable themes.
It was released under the terms of the GNU Lesser General Public License (LGPL).
Development ceased at the end of 2000, when Qt became free software removing the need for the Harmony Project to exist.