The Library of Congress's Primary Source Sets offer a great mix of original drawings, maps, and printed and handwritten documents. What I like about their collection is that each collection includes an explanatory background Teacher's Guide and that each document is available in multiple formats (JPEG, TIFF, or pdf). One set also links to a map where you can find primary sources relating to any particular state. What I don't like is that the sets are presented in alphabetical instead of chronological order (so the set on the Civil War appears before the set on Jamestown).
The National Archives has created DocsTeach, an amazing online tool for engaging students in interpreting primary sources. The documents here are grouped into eight historical eras, which makes choosing the documents for the era you are studying much easier. Also as important, as opposed to the Library of Congress site, DocsTeach lets teachers create online activities where students can manipulate and evaluate them.
This is not an online worksheet! Teachers select a group of documents from their online collection that they want their students to consider, and students then manipulate and evaluate them. For example, in one fun activity called "Weighing the Evidence," teachers offer two differing interpretations of a historical topic. Students consider each document in the teacher-selected document set, then drag each one onto a balance beam on the side that they think the document supports.
This video (2:09) introduces DocsTeach...
and this tutorial (2:44) is a basic introduction to using it.
An updated version of DocsTeach will debut early next year.