There are a couple sites[0,3] which list projects which are looking for contributions. In addition, Github Explore[1] has different projects which might be of interest. If you're interested in larger projects, different companies have guides to getting involved and contributing[2].

[0]: http://www.lookingforpullrequests.com/

[1]: https://github.com/explore

[2]: http://www.mozilla.org/en-US/contribute/

[3]: http://openhatch.org/