KlayGE is a cross-platform open source game engine with plugin-based architecture.

C++ game engine for Windows, macOS, Linux, iOS, tvOS, Android, and web browsers

3D FPS game engine with full dynamic lighting and shadows

NOTE: development in stuntrally3 repo. 3D racing game based on VDrift and OGRE with own Track Editor. The main repository with Stunt Rally 2.x sources and data.

The Dæmon game engine. 20 years of development above id Tech 3. Historically based on id Tech 3 through ioq3, Wolf:ET and XreaL a very long time ago.

Easy to use cross-platform 2D game library for C++

An advanced C++ 3D game engine powering an in-production game yet to be announced

Lightweight game engine.

Game engine playground.

3D racing game with Sci-Fi elements and own Track Editor. The main repository with sources and data. Using Ogre-Next 3.0 and VDrift.

SMK - Simple multimedia kit - C++ WebAssembly

GoldSrc engine plugin for 3D sound

Lost Astronaut's Game Creation Framework: A massive 1800+ class 1400 file extravaganza of OpenGL, WinAPI, OpenAL and many other features, written in Visual Studio for over a decade!

A C++ prototyping engine that is striving for an easy to use API, hiding all the dirty details so that you don't have to care.

OpenAL EAX Extension

A 2D game engine written in C++

Vanda Engine is an open source 3D Game Engine for Windows. It's free for both commercial and personal use.

Igneous is an open source game engine written in C++.

PSeudo - The world's simplest PLAYSTATION emulator

OpenGL 4 PBR engine