Joe and Coolfer both sent in a link to the new George Grantham Bain Collection at the Library of Congress. The collection of about 40,000 images is searchable online, and includes pictures from the 1860s to the 1930s. Search on "New York" and you'll find an amazing gallery of city images-- more than 1100 pictures! The best part is that all of the images are part of the public domain, and available in uncompressed TIFs, in case you want to make prints or use them for an art project.