Ancient Scripts: "A compendium of world-wide writing systems from prehistory to today.
Centre for the Study of Ancient Documents. Includes links to online collections.
Omniglot: "The online encyclopedia of writing systems and language."
Paleography: Reading Old Handwriting 1500-1800. "A practical online tutorial" from the National Archives (UK).
Papyri.info: Dedicated to the study of ancient papyrological documents.
Selections of Arabic, Persian, and Ottoman Calligraphy from the Library of Congress.
Special Collections Resource Center at Syracuse University: Antiquarian Books. Includes rare printed books, medieval manuscripts, cuneiform tablets and more.
Written in Stone: Inscriptions from the National Museum of Saudi Arabia.