Big game source code list – https://github.com/leereilly/games
https://www.unrealengine.com/ue4-on-github – Unreal made a great page describing GitHub and how this all works
https://github.com/showcases/game-engines – games engine source code!
http://www.html5gamedevs.com/topic/6909-list-of-games-on-github/ – some other links to other source code
https://en.wikipedia.org/wiki/List_of_commercial_video_games_with_available_source_code – surprisingly, there is a ton of games that publishers open source once they get old.