I love the Library of Congress, and if you're a writer or a researcher, you should too.
Because not only is the Library of Congress dedicated to preserving some really great stuff. They're dedicated to sharing it--online.
Case in point.
The Library of Congress Map Collection
Need a map of San Francisco from the 1860s? This should be one of the first places you look. Click cities and towns, and type in the place name as the keyword. You'll get a list of maps for the place and the year the map is from.
How easy is that?
And, the Map Collection isn't the only digital collection available. The LOC's Digital Collections contain everything from newspapers to information on dancing styles. Need to listen to a little ragtime to get in the mood of the era? You'll find it.