Currently, I am through a very creative phase as a developer. I think it's a good time to contribute to an open source project. Not as "permanent" developer to a project but in a "help wanted" manner in many projects.
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If you are interested in taking a more introverted approach to contributing to open source projects, then I think Github would be a good place for you to start. You can fork anyone's repository and get your very own local copy. Then you can make the changes you want to make and send them a pull request to get your code back in sync with their project.
All you need to do is find a project that looks interesting, check out their issues page to see if they need help fixing anything, fork, fix, pull request, and that's it!
Also, some larger projects could prefer documentation or unit test contributions if you are interested as well.
The mentioned open-source project sites ares still good. The SourceForge and CodePlex were there for many years, and they are still functional. Some of Git’s predecessors, such as CVS and Subversion, have a central “repository” of all the files associated with a project. The SourceForge and CodePlex, code-sharing sites are using central repository pattern.
However, Git is different. It is build on a distributed version control system design. As it is also mentioned on Git's official website:
In recent years there is a trend to migrate open-source projects to GITHub. Thus, it is becoming the next HOT Spot for open-source community to share their goodies.
In summary, you may get more attention and more projects needing contributors to help.