Godot is a powerful cross-platform game engine
The flexible game engine
Flax Engine, multi-platform 3D game engine
Free game engine for development of desktop, mobile, and web games
Voxel game-creation platform with easy modding and game creation
Recreation of the classic Grand Theft Auto III game executable
Cross-platform 2D and 3D game engine with modular architecture
A complete 3-D game development suite written in Java
Xash3D FWGS engine
A completely free, open-source, 2D game engine
Powerful, mature open-source cross-platform game engine for Python
A fresh & lightweight javascript game engine
Free, cross-platform 2D game engine powered by Haxe and OpenFL
Stride (formerly Xenko), a free and open-source cross-platform C# game
Multi-platform Engine for Desktop, XBOX (UWP) and Mobile games
Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D
Open game and rendering engine packed into a JavaScript framework
Cross-platform game engine in Rust
The Ren'Py Visual Novel Engine
Open source clone of the Age of Empires II engine
Multi Theft Auto is a game engine
The ioquake3 community effort to continue supporting/developing id's
Cross-platform 2D game engine focused on ultimate productivity
The HTML5 Creation Engine
A game engine with an emphasis on real-time cutting-edge solutions