wxWidgets - wxWidgets: Cross-Platform GUI Library
Kissflow - Kissflow is a workflow tool & business process workflow management software to automate your workflow process. Rated #1 cloud workflow software in Google Apps Marketplace.
Electron - Build cross platform desktop apps with web technologies
ProntoForms - ProntoForms is a mobile business solutions application, converting paper forms onto any tablet or mobile device.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Nintex - Cloud-based digital workflow management automation platform