Space Engine - Space Engine is a realistic virtual Universe you can explore on your computer.
Unity - The multiplatform game creation tools for everyone.
Celestia - Real-time 3D visualization of space
Godot Engine - Feature-packed 2D and 3D open source game engine.
Stellarium - Stellarium is a free open source planetarium for your computer.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.