The Digital Library Federation's website, Aquifer American Social History Online, is a site that brings together 175 collections that catalog American social history. Some of the types of materials included on the site are photographs, maps, oral histories, data sets, sheet music, posters, books and journal articles. On the right side of the homepage you can browse by "Times", "Subjects", and "Places". The items included here date back as far as the 1600s, covering the 50 states plus Puerto Rico and subjects ranging from African-Americans to World War II.
http://www.dlfaquifer.org/home