Google Summer of Code 2011 is on. Students, what are you waiting for? Go through last year's pages to narrow down on a mentoring organisation that suits you, and start getting your hands dirty by hacking on some projects as soon as possible. Remember, the earlier you start working on your proposal, the more refined it will be by the application deadline. And if your dream mentoring organisation doesn't get selected, remember that it doesn't stop you from contributing outside of GSoC. :)
You can find all the information you need here.