Open source is a life style. -- Michelangelo Van Dam

Some developers identify open source with weekends spent coding and programs given away for free, but this is not the full picture nor a balanced one. Open source consists not only in contributing to projects with code or documentations, but also in every other activity that provides value to the community: you don't need to patch the PHP core to be active in the open source movement.
For example, in the last year I published a lot of material on my personal blog, including a free ebook on practical testing in PHP applications. As a result of the exposure my articles gave me, I got to be employed as a freelance blogger here at php|architect and to speak at my first conference.
There are also more immediate benefits from writing for the community: if English is not your native language, using it nearly everyday will also improve your proficiency. English is one of the standards of the open source world
and of the business one.
Code
Of course also code can be important: for example, my contributions to Doctrine 1 and 2 gave me credibility when writing about it. We usually can't show our private projects since they are confidential, and certainly a customer won't let you open source his website.
Writing code samples is a mandatory step in the interviews of every place where you would want to work. If your code is out in the wild, it has double value: first, for the users of the project you helped and second, for you to show your skills to potential employers. If you take the step of jumping from the perspective of an end-user of a framework or a library to a contributor or active developer, would you then need a certification? What sounds better on your résumé, that you can work with
Zend Framework, or that you created a part of it?
Moreover, writing open source code makes you learn new patterns and practices, and you can choose what you are interested in so that your work won't be boring. This is one of the factors that makes working on open source attractive where no monetary reward is present: the joy of hacking.
Once you have started coding on open source projects, your contributions can make your name recognized: people dig into source code very often and
@author annotation remains naturally impressed after seeing many of them. Years ago I wondered who this Sebastian Bergmann was.
Open source your career
Thus, participating in the community with code, articles, documentation, and whatever provides value to others can really help your career. Often in subtle ways: the web is open to anyone and you never know who is receiving your content. For instance,
Lorna Jane Mitchell came to know that one of the reasons she stands out during candidate selection and now works at iBuildings was an
Oracle podcast she did for zend.com.
Now she is giving a talk about open source stories, and she's looking for people that are willing to recount to her how contributing to the open source movement have helped them advance in their careers. If you feel community involvement helped you get where you are,
tell her your story.
Image from Wikimedia Commons