You can make it - Open Source MMORPG Platform
Solar2D Game Engine main repository (ex Corona SDK)
Open Source real-time strategy game engine for early Westwood games
The flexible game engine
Fast and lightweight JavaScript game engine built on WebGL and glTF
Godot is a powerful cross-platform game engine
Injectable LUA scripting system, SDK generator, live property editor
A development environment focused on modding
Cross-Platform C++ 2D/3D game engine
A free & open-source 2D sprite editor, made with the Godot Engine
Sandbox video game with a powerful physics engine
Multi Theft Auto is a game engine
A Minecraft mod designed to improve frame rates
A complete 3-D game development suite written in Java
Simple Beat Saber Mod Installer
Open source voxel game engine with support for survival and crafting
The game engine for everyone
Lutris desktop client in Python
Real Time Strategy game engine supporting early Westwood classics
fheroes2 is a recreation of Heroes of Might and Magic II game engine
Dust3D is a cross-platform open-source 3D modeling software
An iOS & tvOS Frontend for Multiple Emulators
Open Source MMORPG Framework
Powerful, mature open-source cross-platform game engine for Python
Open source clone of the Age of Empires II engine