I agree that volunteering confers all of the benefits that Jo outlines in her post. It not only gives you practical experience of working in a library environment but also the chance to use and develop transferable skills, make contacts and build your professional network, gain insider knowledge of the library and sector you're volunteering in, demonstrate commitment to your professional and personal development - the benefits are endless and you can make a lot of the opportunities gained from volunteering.
However, it is important to consider the ramifications of volunteering if the volunteer positions are being used as a substitute for what should be professional paid posts. This may have always been a potential issue but is becoming more so at the moment in the public library sector with cuts to local services and the potential for community-managed and run libraries to fill the gap created in service provision. Johanna Bo Anderson outlines the ethical dilemma members of the library and information profession face when offering their work on a voluntary basis and the comments to the post are thoughtful and thorough. Several people state that while they might be prepared to work voluntarily to provide "added extras" to a service, they would not consider taking a voluntary post that would cover what should be core service provision. I have a lot of sympathy with this view. Local councils (still, just about) have a statutory duty under the Public Libraries & Museums Act 1964 to provide a comprehensive and efficient library service for all and it is unclear at the moment where community-run libraries would fit into this provision (for more about this, see the relevant page of the advocacy section of CILIP's website).
An alternative way to volunteer your time and skills is to get involved in committees and working parties, either at your workplace or in the wider profession, and also to undertake wider voluntary activity with, for example, community or sporting groups and organisations. I was a committee member of my local CILIP branch while an MA student and in the first couple of years of my post-qualification work, and I feel that this gave me useful experience of the wider professional community (while also, I hope, enabling me to give something of value to that community as well). Last year at work, I was part of a team that organised the staff Christmas party (I sort of volunteered and sort of had my arm twisted, although I'm sure I could have said no if I really didn't want to do it). This was quite stressful and time-consuming at the time, but actually it gave me loads of useful things to talk about and evidence of skills - such as team-working, managing a budget and organisational skills - in a subsequent job application. So volunteering, at whatever level and in whatever capacity, does give you the chance to fill in gaps in your experience in ways that might prove to be unexpectedly useful in the future.