There are plenty of opensource projects looking for contribution.
You can check popular SourceForge.net and github.com sites for opensource projects.
Search for a project that of interests you and contribute some code for its improvement/bugfixes/features. You can send a request (email) to the admin of the project. Dont forget to share any of your previous works you had. Or your already contributed code that is a value add.
Also you can start your own opensource project on your interesting topic.
check these links http://www.oss-watch.ac.uk/resources/toptipscommunities ; http://abdullin.com/journal/2010/11/23/contributing-to-open-source-is-so-much-better-than-taking-a.html ; http://arstechnica.com/business/2012/03/ask-stack-how-can-i-find-a-good-open-source-project-to-join/ ; http://www.itworld.com/open-source/80180/building-your-career-open-source hope these helped