FreeOrion
Source code repository of the FreeOrion project
...Built using C++ and Python, it features a flexible game engine and a custom UI tailored to 4X (eXplore, eXpand, eXploit, eXterminate) gameplay. Players manage planets, design ships, conduct research, engage in diplomacy, and wage war across a procedurally generated galaxy. FreeOrion offers a unique tech tree, customizable empires, and supports multiplayer for competitive or cooperative play. It's community-driven, open-source, and under active development.