Giorgio Sironi has a new post to his blog today with a few tips for anyone out there looking to get involved in open source and help out projects you want to work with or already use in your development and want to improve.
Employing an open source project at the user level (in the case of standard applications) gives you a picture of its overall features and maybe an involvement with the supporting community, which is not a deep vision of the project goals and inner workings. But your contribution will be by far more valuable and simple if you start with contributions to codebases you already know "intimately".
Giorgio talks about some of the projects he contributes to (like the Zend Framework and Doctrine) and a few things you can do besides code to help projects out. They're always looking for people to write documentation, user experience feedback and the occasional donation to further the cause.