The Museum of the City of New York has three wonderful photo exhibits up in their online archives: Gotham Comes of Age, Bernice Abbott's Changing New York, and the WPA's Federal Arts Project. For those of us interested in New York street photography from the first half of the century, it's an amazing trove of pictures. Some favorites from each series:

Gotham Comes of Age: Street Life, At Home, At Play, The Physical City
Bernice Abbott: Lower East Side, The Village, Brooklyn, West Side
Federal Arts Project: City Scenes, Mark Nadir, Food, Scenes