Designated state libraries, archives and historical societies exist throughout the United States. As with Presidential Libraries, some of these are official, governmnent-operated institutions, while others are private organisations. The former are charged with preserving the public records of the state, including government documents, and may be found in each state, sometimes linked to a historical society or museum or the public library system. The latter, especially those long-established, often hold significant manuscript and archival collections relating to the history of the state, as well as papers of individuals and institutions or organisations. As well as the state institutions, there are a myriad of local libraries, archives and historical societies with potentially useful archival collections, as well as other university or independent private research libraries.
Most states have also now established digital libraries, designed to bring together digitised materials from institutions across the state. These can be very useful sources of primary documents, particularly of digitisation-friendly material such as photographs, maps, newspapers, letters and diaries, and are well worth checking before considering a trip to the US as, for some locations in particular, a huge amount of older material is now available freely online.
The below institutions have particularly significant collections or whose holdings are broader than just their local area.
The following sources and catalogues can help you identify where papers of individuals and organisations may be held. If your research has a particular geographical focus, the relevant state or local library, archives and historical association are well worth investigating for potentially useful sources.