Multi Theft Auto is a game engine
Descent 3 by Outrage Entertainment
Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D
Open source library for creative expression on the web, desktop, etc.
Go bindings for raylib, a simple and easy-to-use library
The multiplayer game engine
MIT Licensed Open Source version of Torque3D from GarageGames
State Management and Multiplayer Networking for Turn-Based Games
DOOM / Heretic / Hexen source port with enhanced graphics and UI
Amazon Lumberyard is a free AAA game engine
Multiplayer game server based on Node.JS
Turbulenz is a modular 3D and 2D game framework