Below is a list of resources that we have found helpful in our research, and may be useful to others doing historical research.
The New York Public Library Digital Collections are varied archives of historical photos, paintings, documents and more.
Office of the Historian, US Department of State: Collections of selected diplomatic documents from the Truman administration onward.
Foreign Relations of the United States, University of Wisconsin Digital Collections: A large online collection of documents dating back to the Lincoln administration.