Cartophiles rejoice! — the NYPL recently made over 20,000 historical maps from its database free to access, download, and use: http://maps.nypl.org/warper/

A bit overwhelming, but cool nonetheless: it features New York City from 1852 to 1922  as well as world maps, collections from the mid-Atlantic US and the Austro-Hungarian empire, etc. The library also created a tool that apparently lets you overlay the maps on existing images from tools like Google Earth.