This year's list of mentoring organizations is unbelievable; don't say you didn't have a choice. However, let me draw your attention to one of the names in the juicy middle. For those of you who have been working on Mozilla related things with me or just finished taking the Mozilla course, I'd encourage you to consider applying for a Mozilla SoC project.
I had a great chat with one of my students this past week. He was considering doing some Mozilla hacking, but wasn't sure: "I don't really know what I'm doing." I've heard this from others in the past, too. I told him not to confuse 'not knowing how' with 'not being able to figure it out.' As Aristotle wrote in the Niconachean Ethics, the thing that we don't know how to do, we learn by doing it. I also told him about the fun Andrew had last summer implementing APNG. Who could ask for more?
Why would you want to spend your time working on Mozilla? First, you won't find a better community. Learning to make the browser do something it hasn't done before requires more than source code. You'll also want to connect with people who are interested in seeing you succeed and can help you sort through the code base. The number of active members of the Mozilla community helps insure you won't get lost in the shuffle.
Second, unlike many other organizations listed, Mozilla isn't a homogeneous technology. Perhaps you'd like to work on the build system, write an extension, do some web app hacking, go to work on the core, focus on Mac or Windows or Linux, work in C++, not work in C++, etc. --the shear size of this beast, while scary at first, is also what enables so many people with so many skills/interests to collaborate.
At any rate, I would encourage you to come chat with me so I can encourage you more. One thing about open source is that you don't have to work in isolation. There are many people out there who can help. I'm one of them.