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.
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.
FLTK - Fast Light Toolkit - Fast Light Toolkit (FLTK)
Blender - Blender is the open source, cross platform suite of tools for 3D creation.