GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Unity - The multiplatform game creation tools for everyone.
Qt - Powerful, flexible and easy to use, Qt will help you not only meet your tight deadline, but also reduce the maintainable code by an astonishing percentage.
Godot Engine - Feature-packed 2D and 3D open source game engine.
JavaFX - JavaFX is the next step in the evolution of Java as a rich client platform.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.