I know this is a fairly broad question, but I am looking for a resource that lists volunteering opportunities with local charities when travelling overseas or an organisation that does something similar. Sites for local charities probably will list how to get involved; what I am specifically looking for are places that accept short-term (couple of weeks or months) involvement. Not a problem if the opportunities are unpaid (because most tourist visas don't allow employment).
EDIT: Mark's pointed out a pet-peeve: searching online often yields companies that expect payment in return for arranging volunteerships. Not interested in those.