It's already obvious that the benefit of this will greatly depend on the reader of the resume. For example, I don't weight hobby projects lower than professional experience simply because they show a passion for the job that goes beyond money. On the other hand, you may simply be a maverick and not beholden to the job or want to work on your hobbies on company time. It's not clear-cut.
That said, you want to expose them, so do so. I would suggest that you have a section similar to your education history for your open-source projects, and that you include URLs and your userid wherever possible. This makes it possible for the interviewer to verify your code skills before getting to the interview. It shows a level of confidence that can be refreshing. Not arrogance, just confidence. Arrogance is confidence without ability, but you're providing evidence and proof.
Open Source and Hobby Projects
- foo (http://foo.sf.net, userid blah)
- baz (git://baz.git.net, userid blah)
- blurf (webmaster for blurf.org)
- etc.
As for where, well, are these better than your professional experience? If so, put them at the top (most likely, you're a recent grad, so your education is up there, too). If not, put them after your professional experience (most likely, you're not a recent grad, so your education is after the experience section, too).
Just my two cents.