Teach the world to code with CodeCombat. We're a YC W14 company that open sourced everything back in January. Our game teaches complete newbies JavaScript, but we also offer levels for advanced devs. Our goal is to become the game that teaches everyone to program.
We have a bunch of issues for everyone from senior devs to people who don't know how to program over on our repo: https://github.com/codecombat/codecombat
If you are looking for independent projects that won't require as much help from us, you can check out our list of Google Summer of Code example projects: https://github.com/codecombat/codecombat/wiki/Summer-Project...
We also just launched a ChallengePost challenge to encourage contributors to submit transpilers so we can make the game available in different languages: http://codecombat.challengepost.com/
If you have any problems or need help, the founders (myself included) spend most of the day helping people get involved in our public Hipchat room: http://www.hipchat.com/g3plnOKqa
Although code submissions are great, the highest value contribution you could make at the moment is to try our first multiplayer level and give us feedback: http://codecombat.com/play/ladder/dungeon-arena