s&box is a modern game engine, built on Valve's Source 2
05k0nz's legacy is safe
An updated version of the classic "Basic Computer Games" book
3D game engine/framework in C, with Luajit and Python bindings now
A lightweight game server framework implemented with Actor Model
Botcraft is a cross-platform C++ library to create bots
FlightGear Flight Simulator: free open-source multiplatform flight sim
A planetarium software that shows a 3D simulation of the night sky
A MUD client for Android, iOS, Linux, Mac OS X and Windows
Full featured MUD server with all the trimmings.
An ASI plugin for Grand Theft Auto V
A Rust framework for building Minecraft servers
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
A scriptable 2d randomizable maze game
Web powered cross-platform 3D, WebXR game engine
3D-Accelerated open-source DOOM engine
scriptable game engine with built-in support for 2D graphics and sound