Ask HN: Need advice on my next direction

Let's just say I've recently come into a lot of free time. I really want to get down to some intense programming, and my goal is to learn a new language and contribute to an open source project. My background is in Python and Java, and I have little experience with anything else.

My question for all of you is what language? And some potential open source projects that you find interesting and need more contributors. I've been thinking about Go or C++, but throw whatever you think at me.