Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
Zenity - Zenity is a tool that allows you to display GTK dialog boxes in commandline and shell scripts.
Bots UI Kit - Fully customizable Sketch UI Kit for Messenger Platform
Yad - Yad (yet another dialog) is a fork of Zenity with many improvements, such as custom buttons...
Landing UI Kit - Handcrafted UI components for landing pages
Glade - Glade is a RAD tool to enable quick & easy development of user interfaces for the GTK+ toolkit...