MatterControl - MatterControl is a free, open source app that lets you organize and manage your 3D prints.
Unity - The multiplatform game creation tools for everyone.
Slic3r - STL-to-GCODE translator for RepRap printers Here comes Slic3r! Its features are: - speed
Godot Engine - Feature-packed 2D and 3D open source game engine.
Cura - Cura aims to be a end solution for personal 3D printing with RepRap based machines.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.