It's too late for this summer, but if you are still a student next year, then Google's Summer of Code is a great option. Mentoring organizations are usually announced by mid-March (see timeline for this year here http://socghop.appspot.com/document/show/program/google/gsoc...). For almost any interest, there is probably a mentoring organization that is doing something you would be interested in (and if not, google also allows applications for independent projects through their open-source programs office). I've definitely learned as much, if not more, working on a project through SoC as I did at an internship I had at a small software development shop a couple of years ago. The pay is better too. The only downside is that you might spend a lot of time alone, only contacting people through a mailing list and/or irc channel.