This one I am tagging partly for the sake of how entertaining it is. There really are vintage commercials here and you can watch them start to finish. I think these would be applicable in the classroom when talking about how advertising works, etc. or when talking about social culture and how it's changed over time.
This is a site that has primary sources that relate to Greek history. My eye was drawn to this because 6th graders study Greek history and I've noticed that students of all ages seem to be very fascinated by it. I think what this website has would be very meaningful if enough was known about the primary sources being shared. In other words, what I saw that is available here isn't engaging all by itself, but if something was being talked about in a history book or in a piece of literature, then it would be cool to make the connection with a primary source here.
This website is a portal for many other websites that would be very useful for teaching. I haven't looked at all of the links, but there are links to major websites like the Smithsonian and Library of Congress, both of which I know have primary documents available on them. Not all of these sites have government documents on them, but many of them do.
This is another portal with a ton of great links. There is a page full of links for educators and then drop down menus that separate K-6 and 7-12. In these drop-down menus are links to entire pages of websites devoted to topics like the government, history, the military, math, and science. Not all of these sites have government documents, but many of them do.
This website has primary sources and government documents on it. It is another portal site with a section devoted just to kids with links to government information, websites of government agencies for kids, a NASA page, and the health pyramid. It also has primary documents on the census dating back to 1790 which I thought was pretty neat.
This google site allows users to access news stories that have been archived to ones that have posted just minutes ago. Using advanced search tools, users can be very specific in the searches that they do, looking only in certain publications, or limiting searches to include stories only from certain locations, etc.
This website is linked with a lot of information and resources. What I thought was awesome about this site is that it has actual letters written by soldiers and families of soldiers. During a unit on the Revolutionary War, materials like these are priceless and so much more meaningful that words from a textbook.
This website is another portal with many links that are organized very well. This website has 2 links that I would use most often and they are the Ben's Guide to the Government link, which is a very kid-friendly website that talks about the U.S. government, and it has a link to primary early government documents like the Constitution and Bill of Rights, etc. This site may not be as useful in the elementary grades as it would in high school, but the links I mentioned above would be good for 5th and 6th grade students.
This page has countless primary documents and access to so many teacher resources. Nearly any topic you can think of that relates to American History will have some primary source documents available at this site.