I have just recently started to study Ruby, and in lieu of Jeff's advice over the weekend...
...I was interested in honing my skills while helping out the Open Source Community the process so I thought I'd ask if anyone have any suggestions for cool/interesting Open Source Projects written in Ruby that you know of or are involved in.
Well, you didn't specify Rails, so I'm going to through Shoes out there. First, building shoes apps is probably the best way to learn Ruby (Rails is great, but I find mastering Ruby far more fun/useful). Secondly, while I certainly don't think building crossplatform UI components is trivial, shoes is relatively new, and relatively small. There are no doubt countless additions that could be made.
I think if you are attempting to learn a new language do something fun in that language.
I learned python by writing lots of web spiders and little toys and for ruby I'd take exactly the same path. Instead of finding a project that needs input do a couple of little personal projects to get a feel for the language. You learn more by doing something then by reading lots of examples and other peoples code in those first few months.
A language like Ruby is structured in such a way you can do something productive straight out of the box without much support so jump right in and do something fun rather than think you have to do something for somebody else right at the beginning of your experimentation with a new language.
have fun with Ruby, it's on my short list of things to play with :)
The active projects on Rubyforge are a great place to start. What would be a good starter project is to pick one that is pretty popular but not a lot of developers.
If you are interested in Ruby on Rails, I'm working on Redmine right now. It's been one of the most active projects and only has 5 developers. Open Source Rails also has a good collection of projects.
I've found doing a Refactorum a great way to get started on a project. Use the fact that you are new to your advantage, most people who have been on a project forget about simple things like gem dependencies and documentation
Instead of joining an open source project, find an itch you want to scratch.
I find my first year with a language is almost always throw away code (or at least, it should be).
Find a problem you (personally) want to solve. Use ruby to do it. You'll learn a lot.
If you are interested in web development, you could try The People's Feed. It is a very new project but it has a lot of potential =].