National Archives Today's Document feed is a good place to find primary source documents to spark discussion in your classroom. Everyday Today's Document features a new image or document from the archives. The documents are usually accompanied by some additional research links and lesson plan resources.
The same content found in the browser-based version of the Today's Document feed can be viewed in the Android and iOS Today's Document apps. With the mobile app you can select any day of the year to find out what significant things happened in US History on that day. You can also select “random” to have a random document or image appear. The mobile app provides the same background information that you can find online.
Applications for Education
The National Archives Today's Document feed is one that I've followed since the days of Google Reader. I've used the items in the feed as the basis for quick "this day in history lessons." I've also bookmarked many of the documents and pictures in the feed to include as part of larger lessons in U.S. history. For example, years ago I bookmarked a letter from Upton Sinclair to Theodore Roosevelt and used it in a lesson about labor conditions at the beginning of the 20th Century.