Penn State University Libraries’ collection of Pennsylvania Sanborn fire insurance maps dates to 1925, which means that as of this year they’re in the public domain—and freely available to use. Meanwhile, Maps Mania has a roundup of other fire insurance maps resources. The Library of Congress has a collection of 50,000 Sanborn atlases, 35,000 of which are available online (collections, navigator). In the United Kingdom, fire insurance maps were produced by Charles E. Goad Ltd.; Goad maps are available via the British Library and the National Library of Scotland.
Fire insurance maps are an invaluable resource for historical researchers: they’re extremely detailed snapshots of the built environment of virtually every city and town, and there are usually several such snapshots (I’ve seen at least three for my little village, for example), so you can chart a town’s growth over time at a level of detail an OS, quad or topo map can’t match.