That's how much time I spent browsing the David Rumsey Historical Map Collection right after Kurt Hockenbury sent over a link to it. The site is filled with thousands of vintage maps dating from the very beginning of cartography to the early decades of the 20th century. In many cases you can find maps of the same locations over a span of centuries and see the ebb and flow of history over time. Names change, political borders shift around, cities swell and shrink, harbors are developed and silt up...it's fascinating to watch.
From a specifically Lovecraftian standpoint the collection is a treasure trove, featuring vintage maps of Mythos locations around the globe...
...and gems from the classic era like this railroad map of New York City from the 1930's.
Best of all, the export function allows you to download insanely high-resolution scans suitable for printing at full size. The vast majority of the collection is usable as is, but if you're willing to put in a little time adding some appropriate Mythos touches you can create some fantastic prop maps.