I suppose my volunteering began when i was still in secondary school when I was a volunteer librarian in the school library at lunch time. This involved shelving, circulation duties, and other small admin. tasks.
More recently I have been the volunteer / consultant for the Irish Preschool Playgroup Association when that set up their internal library. This included helping set the parameters for the project and the organisation's expectations of it, select a library management system (koha), select a contract librarian to implement the project and get initial data input and organisation for the collection, along with follow up support for the staff subsequent to the librarian's leaving.
Both experiences were good one's for me, and neither were in a position where I was impeding the creation of a permanent post for another librarian. I'm not sure that either of them did anything to further my career, but then again I haven't really had a job interview in a while so I can't be sure one way or another.