@Lou, you're right about Pandas. Wes McKinney is a force of nature. At some point, he just decided that finance needed python, and that python needed a real dataframe library. He then wrote some of the best statistical software ever - high performance, great api, very well documented. He literally wrote the book on python for data, and ran around the country teaching people how to do data work in python. Wes is a hero.
My opinion is that opensource works best when your users are developers, and when the code is a library, rather than a full blown application (firefox is a noteable exception). The opensource trading systems I've seen are not just applications, they are multipart systems: hard to install, hard to run, very hard to develop against. We are opensourcing Zipline, an event-driven simulation library. It installs via pip, and solves the specific problem of running historical simulations in python. We are doing what we can to keep it focused, easy to install, and easy to import into your projects.
Despite opensourcing the most significant component in our system, I don't see us as being in the opensource software business. We are in the algorithmic investment business, and we use opensource software as part of our strategy because we think it is the right way to make the best backtester.
For Quantopian to be a successful service, we need to make algorithmic investing dead simple. We combine our backtester with other libraries like pandas, operate the physical infrastructure to run your algorithms, maintain connectivity with brokerages, and integrate high quality data into a unified environment.
We do all this so you don't have to. We do it so you can put all your energy into the most valuable part: your algorithms.