OpenSiv3D

Clone of the Age of Empires II engine

Desktop/Android/HTML5/iOS Java game development framework

Open Source real-time strategy game engine for early Westwood games such as Command & Conquer: Red Alert written in C# using SDL and OpenGL.

Scalable open-source game backend server: multiplayer, matchmaking, leaderboards, chat, and social features for games.

A Flutter based game engine.

Space exploration, trading, and combat game.