Open source data science projects to contribute
The Julia project is one which I actively contribute to, including the advanced computing and XGBoost libraries. So, I can definitely vouch for it's maintenence and the quality of the community.

Some really good open source data science projects where even the beginners can contribute are:


Sklearn: Always developing at a rapid pace, the sklearn community is always open to new developers and contributors.
H2O: H2O is another fast growing data science projects, working on scalable machine learning and Deep Learning solutions. 
Go: Open source data science road map and resources. Not really a technical project, but is very helpful for absolute beginners and aspiring analysts.
Pylearn2: Another fast growing Machine Learning and Deep Learning project. 
Vowpal Wabbit: The Vowpal Wabbit (VW) project is a fast out-of-core learning system sponsored by Microsoft Research and (previously) Yahoo! Research.


Here is a Quora discussion on such projects and some more which are not mentioned in this answer. 

Here is a another nice discussion about open source Data Science and ML projects in Python.