Open edX, the stack powering edX.org, is a very interesting (and useful!) project to contribute to. The community and the reviewers are very friendly. There are byte-sized tasks available to get started, and you will get guidance on the tickets and PRs. It's a large Django code base, and changes you make impact millions of students within a week - as real and concrete as it gets. : )
https://github.com/edx/edx-platform/blob/master/CONTRIBUTING...
http://bit.ly/edxbugs
http://open.edx.org/
http://github.com/edx/edx-platform/