Ask HN: Which open-source Ruby project to take part in?

I think I know Ruby well enough. Recently, I was thinking about joining to some open-source project to become better as a developer. So now I have to choose which Ruby project to join. Rails is great but I think it's hard enough inside.

Maybe you can suggest me some interesting projects I can contribute? This project shouldn't be a web framework or a web app though. Just some cool command-line app or gem will be fine. Any ideas?