Unity - The multiplatform game creation tools for everyone.
wxWidgets - wxWidgets: Cross-Platform GUI Library
Godot Engine - Feature-packed 2D and 3D open source game engine.
Electron - Build cross platform desktop apps with web technologies
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.