GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
wxWidgets - wxWidgets: Cross-Platform GUI Library
JavaFX - JavaFX is the next step in the evolution of Java as a rich client platform.
Electron - Build cross platform desktop apps with web technologies
Avalonia - A multi-platform .NET UI framework
Uno Platform - Build Mobile, Desktop and WebAssembly apps with C# and XAML.