Map Rectifier is a free online map geo-referencing tool produced by MetaCarta Labs. It is very simple to use. Users upload a map to be georeferenced, then add control points to the original map and a reference map. Georeferenced images can then be downloaded in tif format for use in other applications. MetaCarta Labs “offers experimental tools that are not yet commercially supported, so they may disappear without warning or perform erratically.” A number of georeferenced examples are available online.

By way of Geowanking. See also Fantom Planet and Tanto.