Ask HN: Best open-source projects for first time contributer?

I'm interested in making a start in open-source, but don't have any idea where to begin. I've been programming for about 15 years, mostly for closed-source companies and my own projects. However, I'd like to build confidence in a newbie friendly project before attempting something bigger.

My main languages are mostly in Java and Python, but I don't mind stepping out of my comfort zone.