Two sites to satisfy even the most demanding time travelers.
Dan Vanderkam has taken images from the NYPL's Milstein Collection and geocoded them on Old NYC. That means you can click on any of the red dots in the five boroughs and get an historical street scene of that location. It's an impressive achievement and a great way to stroll through New York history.
Old NYC Photos has a large archive of same, each print of which is available for purchase. 8"x10" prints are $15.99; larger sizes are also available.