Find scholarly articles on your topic. Google Scholar usually only provides citations, so you will want to search the databases for the full text articles.
National Archives An easy to navigate website that has lots of resources. The online catalog can be searched using keywords.
Docs Teach Also run by the National Archives, Docs Teach provides audio, video, charts, maps, graphs and more. The documents are organized according to different periods in American history such as the Civil War and Reconstruction.
Spartacus Educational A great resource for global history. It contains free encyclopedia entries that connect to primary source documents.
Life Magazine Photo Archive Google and Life Magazine have an excellent search engine that lets you search millions of images from Life Magazine Photo Archive.