Applications will open soon for Google Summer of Code, a global program that gives college CS students an opportunity (with a stipend) to do real-work development on open source projects. On Friday, Google announced the 175 mentoring organizations for this summer's program.
You can see the full list here, which will give you an idea of the sorts of projects these organizations are interested in. Moodle, for example, is looking for PHP developers and is interested in report generators and spam filters. Catroid, a programming language for Android inspired by Scratch, is looking for developers to help build out its functionality.
If you're a student interested in applying for the Summer of Code program, you should have a look at this list in order to help you prepare. Even though applications don't open until March 28, you should contact the organizations you'd like to work with now. Many are open to suggestions for other project ideas too.
The Geek Feminism blog has more application tips.