Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
wxWidgets - wxWidgets: Cross-Platform GUI Library
Landing UI Kit - Handcrafted UI components for landing pages
Electron - Build cross platform desktop apps with web technologies
Epic Pxls - UI kits, icons, templates, mockups and more, free
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.