Library of Congress offers some great resources for teachers and students, if you can find them. Fortunately, the LOC seems to be making an effort to make it easier for students and teachers to find useful resources. One such effort is the Student Discovery Sets of primary sources that are available in free iBooks.
There are twelve Student Discovery Sets available as iBooks. Each set is arranged thematically. The sets contain a mix of images, documents, audio recordings, and video clips. Each artifact in each set is accompanied by guiding questions designed to help students analyze what they are seeing, reading, or hearing. Images and texts in the Student Discovery Sets can be annotated with drawing tools built into each iBook.
Applications for Education
The best aspect of the LOC's Student Discovery Sets is the guiding questions that are found within each set. You can use these questions even if you don't have enough iPads or Macs for every student. I would project artifacts from the sets and display the questions as prompts for group discussions in my classroom.