An open source re-implementation of RollerCoaster Tycoon 2 🎢

Flexible level editor

Cataclysm - Dark Days Ahead. A turn-based survival game set in a post-apocalyptic world.

Diablo devolved - magic behind the 1996 computer game

Diablo build for modern operating systems

Industry-standard navigation-mesh toolset for games

Complete Open Source and Modular solution for MMO

OpenTTD is an open source simulation game based upon Transport Tycoon Deluxe

Space exploration, trading, and combat game.

An open source, turn-based strategy game with a high fantasy theme.

Lightweight and modular C++11 graphics middleware for games and data visualization

Written in C++ and using SDL, The Powder Toy is a desktop version of the classic 'falling sand' physics sandbox, it simulates air pressure and velocity as well as heat.

Game engine

Open-source engine for Heroes of Might and Magic III

A fast, scalable, distributed game server engine/framework for C++, include the actor library, network library, can be used as a real time multiplayer game engine ( MMO RPG/MOBA ), which support C#/Lua script/ Unity3d, Cocos2dx and plan to support Unreal.

Third-person Action Roguelike made in Unreal Engine C++. Project for Unreal Engine C++ Course & Stanford University

Third-person Survival Game for Unreal Engine (Sample Project)

Command the forces of The Project in a battle to rebuild the world after mankind has been nearly destroyed by nuclear missiles. A 100% free and open source real-time strategy game for Windows, macOS, Linux, BSD+

fheroes2 is a recreation of Heroes of Might and Magic II game engine.

SuperTux source code