What open source projects would you recommend as a good place for a starting open source developer? Factors that I think would be important are some obvious ones like well written code and a community that is helpful to newbies. But it might be nice if the code base is such that I can start hacking some small problems without really understanding the details of how everything works.
I'd prefer something that can be developed on Linux using C/C++/Java/Python/Scala.
Trying to pick a project like that will never work because it's not something you're passionate about. What's an open source project that you use daily or enjoy using? Go work with that one.
It really depends on what your interests are as to what project to dive into.
Rationale for a larger project (e.g. Firefox, OpenOffice, etc) is that it has many developers, a well established code base, and many small tasks/bugs to be worked out.
Rationale for a smaller project is that you will become more intimate with the code and application. You will likely get to know other developers on the team and understand the overall concept better. Additionally, your additions to the project may be more noticeable.
sourceforge has a list of projects seeking a new developer. Therer are several for your requested programming languages:
http://sourceforge.net/people/?category_id=1
Apart from developers they have more help requests: