Etoys - Etoys is an educational tool for teaching children powerful ideas in compelling ways.
Unity - The multiplatform game creation tools for everyone.
GCompris - Educational Game for Children 2 to 10 years old.
Godot Engine - Feature-packed 2D and 3D open source game engine.
I Know It - "I Know It" is an online game for practicing primary-level math concepts.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.