Volunteering gets a mixed wrap in the job hunt. There is one school of that that volunteering can help you build contacts and experiences to bolster your resume, and another that if you volunteer too much, people will wonder why you never got offered a job. Before we get too far into this, I think volunteerism of any kind is great, and taking the time to give of yourself to others is fantastic, regardless of whether it helps your career. But if you want to use volunteering specifically to help your career in someway, you might want to be careful about how you proceed.
I am currently volunteering at our Science Center. I wanted to find something to get me out of the house while I am otherwise a full time job hunter, so I looked for opportunities that would add to my skills, while taking advantage of my background. The science center is great for that, they love that I am a Scientist, but I predominantly provide "customer service." Trust me, this was nowhere else on my resume. But there is a real shift in attitude when you aren't just feeding figures and data to your boss with candor, but instead trying to figure out how to make people enjoy themselves more. I am liking it a lot. I also get to meet some really cool people, and don't get me started on how awesome it is to play with the exhibits all day. This is good fit for me, because as great as the experience is as a volunteer, I think that it is pretty clear why I am not ever going to get a job offer from them.
In addition, as a volunteer, I have a lot of flexibility. The science center has lots of volunteers; there are old folks who like to sit by the door to greet people and give them maps, and there are high school kids who like to do the demos. Personally, I spend half my day walking the exhibit floors, and half my day working with the Girls Math & Science Program. Right now, I am developing a big event to help introduce girls to careers in Chemistry. I'm working out some demos, I need to solve some problems with rooms, I've got a budget and expect 100 girls in 6 weeks. Again, this is experience that I don't have on my resume- but as a grad student who has had to juggle the logistics of scheduling committee meetings, experiments and teaching, I feel I have the skills to succeed here.
While I feel my experience as a volunteer is adding to my resume, I'm not really that sure it is going to be the magic bullet that lands me a job somehow. I don't think anyone should approach it as if it would be. But it is making it much easier to keep up my enthusiasm and stay creative about my job hunt, so I still think it is worth the time I spend.