The Census Bureau just released reams of data from the 1940 U.S. Census online (the website's been up and down all day; if it doesn't work, try later), and you are going to spend the next hour or so tooling around on the website. I know because I just did it myself.
I learned that in 1940, my two-unit building on Washington Avenue in Brooklyn was owned (at a worth of, ahem, $5,000) and occupied by a single family, the Garbers — Edward and Estella and their children Alvin and Annette — who ran the paint store below (now a bar). Their neighbors on one side were a grocery store manager from Russia named Solomon, his wife Fannie, and their adult son; on the other, an Italian couple, Anthony and Frances Gregoria, and their three children, one of whom, Hope, was a wrapper at a department store.
I could go on. I got lost in these old census schedules this afternoon, wondering about the Maese family down the street — 11 children aged 6 to 27, apparently all supported by their father Joseph's salary as a polisher (?) at Bogel (?) Novelty — and about Solomon's son Louis, making $2,080 (twice as much as anyone else on the block!) as a dress buyer, and still living with his parents to boot.
One thing I wasn't able to do was look up my grandparents. This is because the system is unwieldy, and not yet digitized: it's just hundreds of thousands of high-resolution scans of census rolls, divided by "enumeration district," and only searchable by address (and even then, not quite).
That's okay, though: I'm going to help you cyberstalk your grandparents, or the previous tenants of your building. Here's how.
Enter as much information as you know about the address you're looking for. At the 1940 Census Website, hit "Census Search" in the navigation bar. In the fields to your left, put in as much as you know: the state is required, but to really find what you're looking for you'll likely need county, city and street names as well. If you're in New York City, like me, you don't need to enter your city, just the county information.
Find your enumeration district. All of the rolls in the Census are filed according by "enumeration district" — a number assigned to a specific geographic area that can be anything from a city block to an entire town. If the address you're looking for is rural, or if the street is short, there may only be a handful of results for your search, in which case you can flip through the scanned "descriptions" (i.e., the street or other boundaries) on the Census website to find the district you're looking for. Otherwise, it's much easier to use the map, which you get to using the left-most button in the search results. Once there, hit the "view map" link to bring up the viewer.
This is where it gets annoying. The map viewer sucks. The best way to look for your ED number on a map is by downloading it and using a viewer of your choice to search — something that lets you zoom in and out, ideally. There's a link to download the maps (they're JPEGs), which is nice; the other problem is that some of the maps are several pages long, and there's no way to download them individually. Brooklyn, for example, is 22 pages, and there's no way of telling which page your address will be on. Sorry.
Find your general address, and note the number written on it. The ED numbers are written on your map, as they are here on mine. Since Crown Heights was dense, each block is an enumeration district; more rural areas will have large enumeration districts, and be slightly easier to figure out. My block's number is the barely-legible 1190. Since the Census' search results tell me that Brooklyn is ED 24-1 to 24-2863, I know the full district number is ED 24-1190.
Use the ED number to find your address' Census roll. This bit is much easier: hit the "Enumeration Districts" button and search by state and ED number. Your result should be a single Census Schedule. You can hit the "View" link to bring it up in the Census website's crappy viewer; again, I recommend you download it using the download link — which, thankfully, should allow you to download the entire roll at once after entering a captcha.
Once you've got the Census Roll for the address you're looking for, it should be pretty straightforward: the first column is the street name (usually written vertically across several rows); next to that is the house number. Column 4 tells you if the home is rented or owned and column 5 what the rent or value of the property is. Columns 9-12 are personal information; column 14 the education level; column 15 the place of birth. On the right side, columns 28 and 29 show the occupation and industry and column 32 the individual's salary.
You could spend hours looking around. Find relatives; find former tenants of your building; see what your neighborhood was like 70 years ago. Let us know in the comments.
Update: Ben Vershbow of the New York Public Library labs writes in with a suggestion for another tool you can use:
Hi Max, saw your great guide to searching the 1940 Census on Gawker and wanted to let you know about another resource that you might like to include. I run a small creative technology unit at the New York Public Library called NYPL Labs. In response to the 1940 census release, and to ease the initial difficulty of navigating the census data without street addresses, we digitized 1940 phone books for the five boroughs of NYC and connected them to Stephen Morse's ED-finding tools.
Users can find names directly in the phone books and generate results pages that display ED #s (linking to NARA) alongside fun contextual info like reference maps and a 1940 NY Times headline ticker. Users are also invited to share stories about their searches, adding cultural memory to the search-generated synapses forged between phone book and census record.