wxWidgets - wxWidgets: Cross-Platform GUI Library
Glimmer - Fast and light-weight UI components for the web
Electron - Build cross platform desktop apps with web technologies
Preact.js - Preact is a fast 3kB alternative to React with the same modern API. Components & Virtual DOM.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Svelte - Cybernetically enhanced web apps