Self-hosted game stream host for Moonlight
A free/open source client and automation tool for Ragnarok Online
Python MUD/MUX/MUSH/MU* development system
Open source clone of the Age of Empires II engine
Code for Cicero, an AI agent that plays the game of Diplomacy
A frontend for Mupen64Plus
4X Space Strategy Game
GameServer is a sophisticated, game server designed for online games
GemRB is a portable open-source implementation of the Infinity Engine
Cooperative multiplayer graphical RPG and adventure game
Realism Invictus mod for Civilization IV
Opensource Created Custom Viewers For Virtual Worlds like SecondLife.
... the open-source glide computer
Vivid3D is a C++ IDE/3D engine, with python scripting.
Add and Subtract game
Janken - A Simple Rock, Paper, Scissors Game
The AWS GameKit Plugin for Unreal
DOOM / Heretic / Hexen source port with enhanced graphics and UI
Retro Games in Gym