Most of the time when I'm using this website, though, it's for work on novels that are just coming together in my planning: I have one in mind that's set in the late 1800s on Cape Cod, as the whaling industry waned. It goes with a woman whose life I know the outlines of: married to a whaling man, widowed in her early 20s while expecting her second child, then starting her own business, which (in real life!) would grow to one of the largest in Provincetown. This isn't even on my "wall maps" in my writing room yet (three or four other books are getting written before then!), but I'm filling files with the documents that go with the story. And the new 1940s-era material will help with the second half of the novel.
So much for the notion that writers "just imagine" their stories! Some do ... but for me, the process usually begins with a real moment in time, and a few people whose life decisions at that moment fascinate me.
PS: From the Ancestry folks:
The National Archives and Records Administration will open the 1940 U.S. Federal Census on April 2, 2012—the first time this collection will be made available to the public. Once we receive the census, we will begin uploading census images to our site so the public can browse them. Initially, this collection will be what we call a browse-only collection. This means a person can scroll through the pages of the census districts much like you would look at a microfilm or a book. At the same time, we will be working behind the scenes to create an index of the census that will eventually allow people to search for their family members by name as they currently can with all other censuses on Ancestry.com. Note also that the 1940 U.S. Federal Census will be accessible free of charge throughout 2012 on Ancestry.com.