Godot Engine - Feature-packed 2D and 3D open source game engine.
Unity - The multiplatform game creation tools for everyone.
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Adventure Game Studio - Adventure Game Studio provides the tools to make your own adventure, for free!