"The Devil's Bible" - an on-line illustrated manuscript of a Bible from the Middle Ages. Perfect for demonstrating what an illustrated manuscript is.
Links to a ton of resources, all US history, but a very broad definition of "US History".
Lots of resources here about the history of American spaceflight. Lots.
Nice selection of contemporary issues with facts, figures and like, making preparing for in-class debates a breeze (provided students can read the text.) Non-partisan.
Hundreds of Aesop's fables, listed alphabetically. Plenty of potential uses for these. Some with audio-versions.
Historical Sourcebooks Project: Fordham U
Good resource for world history documents, from Mesopotamia to the post-WWII era
(There is a page ready to post 21st Century documents. There is a single speech from G.W. Bush in 2001 there, waiting for additions.)
Possibly the most famous collection of digitized public domain books on the web. Literally thousands, going back to ancient Greece and Rome.
Popular Science Archives
Great resource for nuclear history, as well as aerospace and other "Popular" science topics. Stories are searchable, and each issue is presented as published including the ads.
National Archives ARC
(Select "Digital Copies" before searching)
Not everything is digitized. If you don't select "Digital Copies" you'll get entry after entry of items you can request from the archives for a fee. Digital copies are available instantly, although some are not very large/high-res.
(Includes "Wayback Machine")
Lists numerous archives and other resources available on the internet.
The "Wayback Machine" is a result of Archive.org taking a "snapshot" of the internet at different intervals, allowing you to access web pages that no longer exist, or previous versions of web pages that are still around.
New York State Archives
This and more at the New York State Quadrant
Smithsonian Magazine Archive
It's not quite as awesome as it sounds, but there's some good stuff there.
New York Times Archive
(Some articles free to non-subscribers)
Life Magazine Photo Archive
Massive collection hosted by Google, and completely searchable.
Hundreds of primary source documents relating to history of "the South." Pretty much every slave narrative written is here, as are documents from the pro-slavery side of the argument. The collection is truly staggering.
Google Art Project
An amazing resource for art. Links to collections listed by museum. Each artwork is captured at astoundingly high-res, and can be inspected closely. Must be seen to be believed.
My only complaint is that I haven't figure out how to download the art for use off-line.
HarpWeek - Harper's Weekly
Public/free version has a significant amount of stuff available from Harper's Weekly's archives from the late 1800s to early 1900s, including hundreds of political cartoons (with lengthy explanations). Students/teachers/professors with access through school, or paid subscribers can access the full archives.
Not an "archive" as such, but a collection of dozens and dozens of free lectures by amazing scholars on a wide array of topics, from science to religion, history, mathematics and more. Great resource for teaching yourself.
YouTube WWII Propaganda Videos
Significant collection of really engaging videos. Occasionally care must be taken due to the racist imagery that was common in reference to the Japanese at the time.
Dickinson College Digital Collections
A ton of resources, many dealing with antebellum America. Dr. Matt Pinsker is one of the organizers, and is one swell guy!
Dr. Suess Went to War
Complete collection of political/editorial cartoons by Theodore Geisel as seen in PM magazine in the early 1940s. Covers almost the entire war, include numerous cartoons about isolationist America
Free e-books, lots of free e-books
MIT Visualizing Cultures
East Asian (mostly Japanese) images on a variety of topics.
Census data from 1790 to the present (recent census data is limited for privacy reasons.)
Paintings of American families - really useful for good social history.
Library of Congress Teacher Resources
Includes a number of document study guides for a variety of different document types.
One of the gentlemen at today's presentation showed me this one on his smart phone. I just looked at it briefly, and it appears to have an absurd amount of information (which is good.)
Wisconsin Historical Society
Plenty of resources for teachers here. Of especial interest are the "Classroom Tools" and "Primary Sources and State History" (Check in "American Journey - primary sources of American travelers from Eric the Red and more)
Stanford History Education Group
Terrific collection of lessons and documents, some scaffolded for weaker readers.