Powerful, mature open-source cross-platform game engine for Python
Mangos ZERO is a server for World of Warcraft
Self-hosted game stream host for Moonlight
Open source clone of the Age of Empires II engine
A free/open source client and automation tool for Ragnarok Online
Source code repository of the FreeOrion project
Code for Cicero, an AI agent that plays the game of Diplomacy
Doom-based AI research platform for reinforcement learning
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
4X Space Strategy Game
Cooperative multiplayer graphical RPG and adventure game
GemRB is a portable open-source implementation of the Infinity Engine
For exploring cellular automata like Conway's Game of Life.
Realism Invictus mod for Civilization IV
... the open-source glide computer
GameServer is a sophisticated, game server designed for online games
Janken - A Simple Rock, Paper, Scissors Game
Real-time collision detection and multi-physics simulation for VR
Opensource crossplatform Fallout 2 game engine writen in C++ and SDL
Procedurally-Generated Game-Like Gym-Environments
DOOM / Heretic / Hexen source port with enhanced graphics and UI
KlayGE is a cross-platform open source game engine