nemozone

Below is a list of open source games and game-related projects that can be found on GitHub – old school text adventures, educational games, 8-bit platform games, browser-based games, indie games, GameJam projects, add-ons/maps/hacks/plugins for commercial games, libraries, frameworks, engines, you name it…

https://github.com/leereilly/games

This is also nice:

http://www.lebottindesjeuxlinux.tuxfamily.org/en/home-en/