Apparently I forgot to hit publish on this post, oops.
RPGs are no strangers to the use of maps, many systems utilise them during combat while adventures often employ them to provide players with a feel for the regions they’re exploring and acting in. One of the downsides of maps is the amount of time that can be required for their construction, especially if you want something that incorporates a substantial amount of detail. Luckily for us then that the Lionel Pincus and Princess Firyal Map Division of the New York Public Library has released 20,000+ historical maps under a Creative Commons CC0 1.0 Universal Public Domain Dedication (which basically waives their copyright to the fullest extent possible). The maps are available are high resolution downloads and while the majority are focused upon the New York region of the USA there are still hundreds if not thousands that span the wider world.
The freebies don’t stop there however. As part of their attempt to digitise the maps the library have also released a tool called the Map Warper which allows the public to align and overlay the historical maps with the location as it exists today using. The merge of historical and present day maps can be exported as basic images or even as Google map readable files, allowing GMs to then add game related details before making them available to their players (see this overlay as a basic example).
Already I can see it’s potential in games such as the Vampire Giovanni Chronicles that Mike has talked about in the past where the action jumps across multiple time frames. Or alternatively how about a modern Cthulhu game where the players come across an old manuscript from the 1920’s? You only need to look at the classic gamecube game Eternal Darkness: Sanity’s Requiem to know how awesome that could be. So if you run a game set in the real world then I would encourage you to check out this amazing resource and if you do let us know about your experience with it.