Desktop/Android/HTML5/iOS Java game development framework
A cross-platform PSP emulator
Open source simulation game based on Transport Tycoon Deluxe
Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D
Open Source MMORPG Framework
An Open Source DOS emulator to run old DOS games
FlightGear Flight Simulator: free open-source multiplatform flight sim
RTS game with the theme of Strategy by Design
A strategy game with a high fantasy theme
Chess Database and PGN viewer
Good-looking and addictive OpenSource games
For exploring cellular automata like Conway's Game of Life.
Odamex is a free, cross-platform, modern client-server Doom engine
Nestopia UE
Seven Kingdoms: Ancient Adversaries