Images
American Memory Historical Photo and Print Collection
Creative Commons Photos
Behold: Best of Flickr
LIFE : photo archive hosted by Google
Finding Images and Sounds on the Web
Library of Congress Prints and Photographs Online Catalog
Library Spot: Images, Clip Art, Search Engines, Collections
The NYPL Digital Gallery
PhotoGraphic Libraries
Wikipedia: Free Image Resources
Copyright-Friendly and Copyleft Images and Sound for Use on Media Projects - from Joyce Calenza's wiki
Music
ccMixter: A community music remixing site featuring remixes and samples licensed under Creative Commons licenses.
Freeplay Music: A comprehensive collection of both musical offerings and sound effects.
Soundzabound: Royalty free database featuring a wide selection of music and sound effects. Located under Special Collections on the bottom right of the database page.
Video