The Ren'Py Visual Novel Engine
Cross-platform 2D and 3D game engine with modular architecture
Stride (formerly Xenko), a free and open-source cross-platform C# game
Cocos simplifies game creation and distribution with Cocos Creator
Multi-platform Engine for Desktop, XBOX (UWP) and Mobile games
Games you can play in your terminal written in go
Cross-Platform C++ 2D/3D game engine
A completely free, open-source, 2D game engine
A free/open source client and automation tool for Ragnarok Online
Cross-platform, open source, and super fast MUD client with scripting
Free game engine for development of desktop, mobile, and web games
Voxel game-creation platform with easy modding and game creation
Kitty-Tools is a Kahoot Cheating Client for all devices and interfaces
An OpenGL RTS game engine written in C
Source code repository of the FreeOrion project
A free and open-source Touhou Project fangame
A lightweight game server framework implemented with Actor Model
Open source clone of the Age of Empires II engine
3D game engine with lua scripting
Botcraft is a cross-platform C++ library to create bots
FlightGear Flight Simulator: free open-source multiplatform flight sim
3D modeler, 3D game maker, 3D demo maker
3D C++ Game Engine - yet another open source game engine
2D Game Engine
A lightweight, fast and extensible game server for Minecraft