Open Source Game Engine
A game engine is the foundation you build years of work on, which makes its license the most consequential technical choice a studio makes - a per-install fee or a sudden change to the royalty terms can claw back the economics of a finished game after you've already shipped it. The open source engines here come with no royalty, no seat tax, and no upstream company that can revise the deal under you, so the full toolchain, source, and runtime stay yours to fork, retarget, and ship without asking permission.
Godot Engine
Free, open source 2D and 3D game engine with one-click exports to desktop, mobile, web, and consoles
Bevy
Data-driven 2D and 3D game engine built in Rust around the Entity Component System
Phaser
Open source HTML5 game framework for building 2D games with WebGL and Canvas rendering
libGDX
Cross-platform Java game development framework for Windows, Linux, macOS, Android, web browsers, and iOS
OpenRA
Libre real-time strategy game engine for classic Westwood games, with modernized play and mod support
MonoGame
Open-source .NET framework for building cross-platform games in C#
Luanti
Open source voxel game engine for easy modding and game creation
Open 3D Engine
Open-source, real-time 3D engine for building games, worlds, and high-fidelity simulations
Stride
Open-source C# game engine for 2D and 3D games, with realistic rendering, VR, and a visual editor