Fuse - Fuse is a UX tool suite for mobile app designers and developers.
wxWidgets - wxWidgets: Cross-Platform GUI Library
Dropsource - Mobile development platform for building native iOS & Android apps
Electron - Build cross platform desktop apps with web technologies
Siberian CMS - Siberian is an Open-Source and Free App Maker. Unlimited Push Notifications. Unlimited features. Fully Customizable. Download it and build your own app now!
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.