Yad - Yad (yet another dialog) is a fork of Zenity with many improvements, such as custom buttons...
Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
Glade - Glade is a RAD tool to enable quick & easy development of user interfaces for the GTK+ toolkit...
Bots UI Kit - Fully customizable Sketch UI Kit for Messenger Platform
wxFormBuilder - wxWidgets is an excellent framework that enables the creation of multi-platform applications with...
Landing UI Kit - Handcrafted UI components for landing pages