WordPress - WordPress is web software you can use to create a beautiful website or blog. We like to say that WordPress is both free and priceless at the same time.
wxWidgets - wxWidgets: Cross-Platform GUI Library
ApostropheCMS - Apostrophe is a powerful website builder platform built on an enterprise open source CMS, offering in-context live editing, visual design tools, and multisite enablement in a full stack JS environment.
Electron - Build cross platform desktop apps with web technologies
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.