TripleA - TripleA is a free (under GPL) turn based strategy game based on Axis & Allies board game.
Unity - The multiplatform game creation tools for everyone.
Warlight - Warzone is a customizable Risk-like strategy game where you compete with your friends to conquer the world.
Godot Engine - Feature-packed 2D and 3D open source game engine.
Domination - Domination is a game that is a bit like the well known board game of Risk.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.