Via Twitter just now, two archival resources with great visual materials:

1. Turn of the Century Posters at the New York Public Library http://images.nypl.org/index.php?id=1543077&t=f

2. From the New York Public Library, an interactive map of New York over time

http://mgiraldo.github.io/scrollnyc/?utm_campaign=SocialFlow&utm_source=twitter.com&utm_medium=referral