Good open source projects to contribute to?

I am an intermediate level developer with background in PHP, but I am moving towards RoR development now. I would love the opportunity to work on some open source projects, but I don't have a good grasp of where to start in the Ruby community. I'm by no means an expert Rubyist either, so I am looking for something I can handle on my own level, while still contributing something useful. I'd appreciate any suggestions or advice!