Godot Engine - Feature-packed 2D and 3D open source game engine.
OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.
Synfig Studio - Just another WordPress site
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.