The Library of Congress Digital Collection is a wonderful resouce for primary sources. Note the field on the left of the webpage. You can search by
If you use the dropdown menu at the top, you can switch from Digital Collection to Personal Narrative. In personal narrative, you will find interviews with people, and in particular a large selection of interviews with veterans.
Local histoical societies often have online resources that can be helpful for researchers. These links are for local (Maryland and Montgomery County) societies. If you are researching a topic from a different area, you should see if there is a historical society with online collections.
The Maryland Historical Society has a rich array of online research resources. Click here to access.
Montgomery County also has an historical society with useful information for researchers. Click here to access.
ProQuest's Historical Newspaper database has full coverage of 36 newspapers going back into the 1700s, most articles are from the 20th century, however. Newspapers include The Washington Post, The New York Times, The Wall Street Journal, and other well known publications. Depending on your topic, you might prefer a regional newspaper, such as The Atlanta Journal or The Chicago Tribune.
Photo by beanworks at Morguefile.com, used under creative commons license.
National World War I Museum and Memorial has extensive digital collections of primary source documents.
The United States Holocaust Memorial Museum has over 200,000 artifacts, films, photos, and more which are searchable through the this website. It also includes the Holocaust Encyclopedia which provides background information on the Holocaust.
The National Archives have a rich set of data, but it might take a little more digging to find. You can search in a search bar, but also within collections such as War and Military Records, Founding Documents, specific places or subpopulations such as Women.
The Primary Source Sets of the Digital Public Library of America (DPLA) are curated around specific topics. Each set contains numerous primary sources including images, music, interviews and more. There are many, so you will need to scroll through to see if your topic is included.
The University of Washington Libraries has an amazing and extensive selection of primary source resources. You will need to look around a bit, but there are a tremendous number of specialized sources listed here!
The University of Maryland archives have an extensive collection of materials on a wide range of topics including African American HIstory, Business and Labor history, Agriculture, Women's History, and much more. Click here to access.