Open Source projects offer you a chance to work on something that you want rather than something that others want you to work on. It is a great opportunity to work on something that is both cool and useful as well as to associate with a well known brand and all the publicity and goodwill it brings. You are free to pick and choose between the thousands of open source projects out there. Moreover, you are free to decide on how much you want to contribute. You won't have a boss and you won't have the pressure of deadlines and schedules.
And don't forget there are about a thousand different ways to contribute to open source projects, with documentation/wiki contributions typically being the most neglected. So find a project you love and get your hands dirty! ALL open source projects can use the help, and it's a great way to learn new things, meet new people, and boost your resume all at the same time. As Michael Scott would say, "It's Win Win Win." ;-)