Qt and Qt3D. I haven't contributed as much to Qt as I've wanted to, but the little I did was met with support and great mentoring. They are a very talented group.

If you're interested in physics, mathematics or other sciences, I'd recommend SciPy. I attended one of their conferences and met many really nice, kind and talented people. (Open Source conferences and meet-ups are a great way to get to know people and kick-starting contributions.)