The National Archives is a fantastic resource. Their website is easy to navigate and includes lots of teacher resources. They feature a daily historical document relating to an event from that day in history. The online catalog can be searched using keywords, and 100 "milestone" documents are identified as significant to American history.
Also run by the National Archives, DocsTeach is full of activities for educators. The documents are organized by different periods in American history. If you're teaching "Civil War and Reconstruction" or "Revolution and the New Nation," just click on the topic to find hundreds of primary source documents. DocsTeach provides audio, video, charts, graphs, maps and more.
Library of Congress
Life Magazine Photo Archive
Google and Life Magazine have a wonderful search engine that lets users search millions of images from the Life Magazine Photo Archive. Not only can you type in key terms to guide your searches, you can also look through images organized by decade (1860s through 1970s) or significant people, places, events or sports topics.
Digital Public Library
The Avalon Project
Broken down by time period then listed in alphabetical order, the Avalon Projectat Yale University also has primary sources for global history teachers. This database starts with ancient and medieval documents and moves into present times. In addition to categories that address specific historical periods, the Avalon Project includes links to human rights documents as part of Project Diana.