Self-hosted game stream host for Moonlight
Open source clone of the Age of Empires II engine
Powerful, mature open-source cross-platform game engine for Python
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
Open Source MMORPG Framework
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
Mangos ZERO is a server for World of Warcraft
Doom-based AI research platform for reinforcement learning
4X Space Strategy Game
For exploring cellular automata like Conway's Game of Life.
A LinuxSampler Qt GUI Interface
Cooperative multiplayer graphical RPG and adventure game
GemRB is a portable open-source implementation of the Infinity Engine
Realism Invictus mod for Civilization IV
Opensource Created Custom Viewers For Virtual Worlds like SecondLife.
A fun realistic multiplayer free-and-open-source first-person-shooter!
Vivid3D is a C++ IDE/3D engine, with python scripting.
... the open-source glide computer
GameServer is a sophisticated, game server designed for online games