Gemrb

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

Clone of the Age of Empires II engine

Community effort to maintain and improve Jedi Academy (SP & MP) + Jedi Outcast (SP only) released by Raven Software

A game engine made as part of the Kohi Game Engine series on YouTube (and Twitch!), where we make a game engine from the ground up using C and Vulkan.

C++20 framework for creative coding / Cross-platform support (Windows, macOS, Linux, and the Web)

Open-source, cross-platform 2D/3D/multiplayer game engine designed for everyone.