Self-hosted game stream host for Moonlight
Powerful, mature open-source cross-platform game engine for Python
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
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.
Realism Invictus mod for Civilization IV
Opensource Created Custom Viewers For Virtual Worlds like SecondLife.
Cooperative multiplayer graphical RPG and adventure game
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
Janken - A Simple Rock, Paper, Scissors Game
Real-time collision detection and multi-physics simulation for VR
Procedurally-Generated Game-Like Gym-Environments
DOOM / Heretic / Hexen source port with enhanced graphics and UI