Inqlude provides a comprehensive listing of all existing libraries for developers of applications using the Qt toolkit. Inqlude is run by the community and open for contributions.
QtQuick plugins to build user interfaces based on the KDE UX guidelines
Kirigami is a set of QtQuick components at the moment targeted for mobile use (in the future desktop as well) targeting both Plasma Mobile and Android. It’s not a whole set of components, all the “Primitive” ones like buttons and textboxes are a job for QtQuickControls (soon QtQuickControls2) but it’s a set of high level components to make the creation of applications that look and feel great on mobile as well as desktop devices and follow the Kirigami Human Interface Guidelines.