And aside from the many good things Bryce listed, there's another way working on Open Source can help one's "regular" career: resumés and job interviews.

Most often, a programmer won't actually be able to show his work product (as opposed to say a graphic designer who has a full portfolio of artwork done for clients). Work on an open source project can be used to show potential employers samples of the quality of your coding. And some may even have heard of projects and involvement. At the least you have something that's legally safe to share by showing others.

Then if the potential employer has some idea about open source, you might be asked "So, if I go look I'll seen your name all over the changelogs?". Being able to answer with where to look (even if it's "ah, go look here on these Wiki pages for design work I've done") can definitely give you a step up. Also it's another way for them to spot the resumé padders. And aside from showing coding samples, it can also be a good way to show how you work in a group. Remember, employers are usually working for people to work together in the company, so the "social" aspects can give a candidate an advantage in getting hired.