Godot is a powerful cross-platform game engine
A complete 3-D game development suite written in Java
Xash3D FWGS engine
Cross-platform 2D and 3D game engine with modular architecture
Cross-Platform C++ 2D/3D game engine
Powerful, mature open-source cross-platform game engine for Python
Voxel game-creation platform with easy modding and game creation
Cocos simplifies game creation and distribution with Cocos Creator
The HTML5 Creation Engine
Recreation of the classic Grand Theft Auto III game executable
Stride (formerly Xenko), a free and open-source cross-platform C# game
Cross-platform 2D game engine focused on ultimate productivity
The flexible game engine
Multi-platform Engine for Desktop, XBOX (UWP) and Mobile games
Cross-platform game engine in Rust
The ioquake3 community effort to continue supporting/developing id's
Zig game engine & graphics toolkit
Descent 3 by Outrage Entertainment
The Ren'Py Visual Novel Engine
A completely free, open-source, 2D game engine
Free game engine for development of desktop, mobile, and web games
3D Procedural Game Engine Using OpenGL
Voxel game engine in C++ with OpenGL
Open source clone of the Age of Empires II engine
Open source library for creative expression on the web, desktop, etc.