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