Direct Me NYC 1940 helps researchers marry the 1940 census with the digitized phone books of all five boroughs of New York City, using Steve Morse's One-Step site.
There's a great article up at the New York Times about it.
The project makes it possible to cross-reference NYC residents’ names and addresses with the 1940 census data made public last year, which includes the person’s age, income, education, occupation, and residences in 1935 and 1940.
There's a place for users to share their discoveries, and as usual, Thomas MacEntee has been here before us. The site also provides context for your finds with a streaming banner of 1940 news from the NY Times archives.
"The result," according to the New York Times, "is serendipitous and eye-opening for scholars and family genealogists searching out their roots." I agree: the user interface, instructions, and contextual results are beautifully done.
Now if some other big cities (I'm looking at you, Chicago) would follow suit.