Some of Drupal's Summer of Code success stories include:

Angela Byron (webchick) the Drupal 7 co-maintainer, Director of Community Development at Acquia, a Google-O'Reilly Open Source Hall of Famer and a Drupal Association board member. She originally got her start in Drupal writing Quiz module for GSoC 2005.
Sumit Kataria, started as a GSoC student back in 2009 working on OAuth module, and now not only is one of the foremost experts in the Drupal community on mobile (look for his mobile apps for DrupalCon Denver in an app store near you!), but co-manages Drupal's involvement in GSoC. He works as a Drupal consultant with companies like CivicActions and Lullabot.
Bojan Zivanovic (bojanz) became a preeminent contributor to views and contributed to EntityFieldQuery for Drupal 7.
Gábor Hojtsy, the co-maintainer of Drupal 6, and the Initiative Lead for the Drupal 8 Multilingual Initiative worked over GSoC in 2006 to get i18n in Drupal core in Drupal 6. He is now an engineer for Acquia.
Jimmy Berry (boombatower) was instrumental in the development of Drupal's automated testing framework, and he and his father Jim Berry (solotandem) were the first Google Summer of Code father/son team! :) They both offer testing-related services at http://boombatower.com.
Lin Clark (linclark) created SPARQL Views, making it possible to query SPARQL endpoints from Views, as part of GSoC 2010. Her demonstrations of Linked Data capabilities in Drupal have been published on IBM Developer Works. She is now an independent consultant working data publishing and consumption using Drupal.

So if you're:

• a post-secondary student looking for an exciting project with a thriving development community and tons of smart people you can work with
• an existing Drupal contributor who happens to be attending college/university and would love a chance to get paid over the summer to work on the "Next Big Drupal Thing"
• a seasoned Drupal developer with some time over the summer, who'd truly enjoy mentoring and helping the next generation of contributors make Drupal the best that it can be
• a Drupal community member who might not have the time or coding experience to mentor, but knows where to find resources and enjoys helping others find them.
• someone with a great project idea for an improvement in Drupal that would be perfect for a student to work on over the summer
• a Drupal evangelist who wants to help grow the community by actively engaging students

...then there's something for you in Summer of Code! Read on to find out more.

Prospective Students

If you have enthusiasm the drive to work on something great, now is the time for you to get started! Subscribe to the Google Summer of Code group, look over the developer's guide and API reference, stop by Core Office hours and take on some new contributor tasks, find a Drupal event near you to get to know Drupal's amazing community, and take on a few bite-sized tasks in the Novice Issue Queue.

Most importantly, start thinking about your project proposal! Prior to submitting your application, stop by #drupal on irc.freenode.net or post your project ideas to the Summer of Code 2012 group to get community feedback. Your chances of getting into Summer of Code increase if the community has the opportunity to review your ideas and offer feedback to help you in improving your project idea.

We have already started accepting applications. For more tips, students should check out the Student Template Page.

Mentors

Please sign up to be a mentor if you have either experience with Drupal development or expertise in a particular area of interest (for example, newspapers, education...) and have some free time from now until the end of August.

To become a mentor,  join the Drupal SoC-2012 group and the sign up on Google's SoC mentor web app (now known as Melange). Please describe who you are, what your level of Drupal experience is, and your motivation for being a mentor. Your application will be reviewed by SoC admins (Chx, SumitK).

You can go through Advice for mentors page to find more tips on mentoring students.

The more mentors we have, the more students we can get in, and the more exciting projects of varying types we can accept.

Community members

Great project ideas are vital to attracting both great students and great mentors. If you've ever thought "if Drupal could be...", now is the time to turn it into a project idea. The project should be feasible for a Drupal-novice developer student to achieve in a 3-month time frame. Suggest a SoC project idea in the SoC 2012 group or help elaborating already proposed ideas

In addition, you can help review the existing SoC project ideas by providing students and other community members with feedback. Community members are in the best position to help students understand the finer intricacies of existing modules, and help their energies to meet the the priorities of the Drupal project.

To help the new Drupal family members, we need some existing community members to be active in #drupal-contribute on irc.freenode.net to answer student questions, point them to the correct resources, and people with expertise.

If you think this sounds like fun, be sure to get on to IRC!

Drupal version: Drupal 7.xDrupal 8.x