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
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
Doom-based AI research platform for reinforcement learning
4X Space Strategy Game
GemRB is a portable open-source implementation of the Infinity Engine
For exploring cellular automata like Conway's Game of Life.
Cooperative multiplayer graphical RPG and adventure game
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