This site has information for teachers and students how to best use the national archives. It has a link to the archived documents. I liked how it had a map of the United States and you could just click on Nebraska and it pulls up all the primary documents it has about it.
This site is arranged by historical eras. It is easy to also search by keywords. Its purpose is to help student develop historical thinking skills. It also has a creation tool where teachers can make their own interactive activities to go with the documents. To access this you do have to create an account.