There are plenty of jobs that are 100% open source. Based on my experience working on open source software is both interesting and challenging. Biggest challenge for me is that people are expecting you to provide 24/7 support.
Here are some jobs I found that are 100% open source:
http://careers.stackoverflow.com/jobs/80088/javascript-engin...
http://careers.stackoverflow.com/jobs/80212/senior-software-...