So far I haven't really done any volunteer work in libraries. It's something I would be quite happy to do to get experience if I was missing a criteria from a dream job's description, however I would always check that I wasn't taking a job away from paid qualified staff. I think there are a lot of ways that volunteers can do a lot of good for libraries though. I had a quick look around a Volunteering Fair at UCL yesterday, and Westminster Libraries had a stall there recruiting volunteers for a homework club scheme they run across the area. Running this kind of thing would take up a lot of staff time, and a library degree really isn't necessary to help kids with their maths homework, so this is a nice way to use volunteers to supplement core library services.
Okay so what have I done? I have done the odd bits and pieces of voluntary "stuff" such as lending a hand occasionally as an assistant Cub Scout leader, organising Cam23 2.0, and I've recently joined the Cambridge Library Group committee. Things like this always seem to be very useful things to be able to talk about in job applications and at interviews, as well as being good fun to do!
PS: Since I wrote my Thing 21 post about job applications and interviews, the summary has gone up for the #uklibchat on LIS Jobs and Careers, which is well worth a read.