There is some really fantastic stuff here. I'm especially excited about the maps, American[1] and otherwise, but the stereoscopic views[2] are amazing. They really give you a feel for what it must have been like to view these things. A fun technological trip.
1: http://digitalcollections.nypl.org/collections/5cd94760-c52a... 2: http://digitalcollections.nypl.org/collections/robert-n-denn...