I work on LiteCommerce 3, a e-commerce open-source project written in PHP, and we are looking for contributors who could extend its functionality by developing cool modules.
Why this may be interesting for you:
1. There is Developer Challenge incentive program with prizes totaling $20000 for the best modules
2. You can sell your modules through our Module Marketplace
3. It's a great opportunity to gain experience in using modern coding practices and techniques: Doctrine 2.0 ORM framework, advanced MVC, OOP (PHP 5.3), the innovative, extremely modular architecture (a module can alter any base class, not just extend it), and so on.
4. We host the source code at GitHub, so you can fork it and send us pull requests with your improvements.
I would be happy to answer any your questions there.