I've always been a fan of the graphic design styles related to wartime propaganda posters, WPA posters, and World's Fair posters. While conducting graphic research for work, I stumbled upon an archive of WPA posters in the Library of Congress' website. Despite the fact that a lot of them have to do with syphilis, the posters are very iconic and historically essential. It's very common to see modern designers drwaing inspiration from this graphic era.

Below is a sample collage of posters:

The website contains downloadable high and low resolution versions of hundreds of WPA posters and important historical data about them. Most of the posters have no known rights restrictions.