Google is running their Summer of Code (http://socghop.appspot.com/) program again this year, and Horde will apply to be a mentoring organization. This page is mainly for those with project ideas and those interested in participating as a student. If you'd like to help out by mentoring, send a note to Horde's mentor coordinator, Jan Schneider (jan at horde dot org).
We have quite a number of project ideas ranging from entire Horde applications to framework libraries, to adding additional functionality to existing applications. For a list of our project ideas, see our Project page. Also remember that if you have an idea that is not listed, it doesn't mean that it's not an option! Feel free to catch up with us on the mailing list or the #horde IRC channel on freenode.net to discuss your ideas. You can also look at our list of open enhancement requests (http://bugs.horde.org/query/enhancements) for further ideas.
If you are interested in participating as a student, please join our development mailing list (http://lists.horde.org/mailman/listinfo/dev) and say hello!