I'm a diligent contributor to gobs of projects on Github, only my most favourite are listed below. A few of them are at present closed source but will someday be made public.
A Final Fantasy XI server emulator. The game is played by hundreds of gamers, 100+ contributors and I'm one of the oldest contributors with over 700 commits.
A javascript entity-component game engine. Entity component design is a paradigm shift in game engine architure, allowing logic to be easily attached to actors in a game with little maintenance overhead.
A Flash game studio I started many years ago. Created the Modern Tactics series and other spin offs. The studio is no longer active but all the games can stil l be played.