I enjoy contributing code to open source projects, and fortunately I actually do that for a living. Here are the ones I am most proud of:

scikit-learn

David Cournapeau started this project in 2007. I took over the project in January since 2010, made the first public release the 1st February 2010 and have been developer, maintainer and release manager since. [Webpage]

YEP

Yep is an Extension Profiler. More info: http://pypi.python.org/pypi/yep

SymPy

SymPy is a Computer Algebra System written in Python. Originally written by Ondrej Certik, I was the first contributor to join the project. I’ve made important contributions to this project in the period 2007-2009, including a Google Summer of code scolarship in 2010 to improve the logic module.

SciPy

I have contributed some code to improve the linear algebra module, including the `linalg.solve_linalg` function introduced in 0.9 and some improvements to orthogonal decomposition to appear in 0.10.

Repos

Most of my code can be found in Github