NYC-Architecture is an interesting site with photographs of the Lower East Side, both period and contemporary.
There are also extensive photo archives organized by neighborhood for Manhattan and Brooklyn, as well as for bridges, walking tours, postcards, etc. The site is searchable by style, architect, type, area and alphabetically.
Informative histories of Brooklyn and its neighborhoods are fun to read, and there's even a section on the beautiful mansions of Newport, Rhode Island, and the town's history.
And if you haven't had enough images to wax nostalgic over, there's always Brooklyn Collectibles, with even more Old Brooklyn photos organized by neighborhood.
The company offers high-quality photographic views of Brooklyn and New York City neighborhoods, from 1890-1965. Author and local historian Brian Merlis has thousands of images of theaters, high schools, Coney Island, Ebbets Field, trolleys, railroads and subways, stores, and regular street scenes.
While it is a commercial site and sells its photos, the many images available are to kvell over.