Open content

Clipart
 * WPClipart
 * Open Clip Art Library
 * Wikimedia Commons

Photos (Do an Advanced search for CC-licensed photos)
 * Flickr
 * Wikimedia Commons
 * The Open Photo Project
 * Image*After
 * morgueFile
 * Stock.XCHNG
 * ookaboo
 * NASA
 * Library of Congress

Maps


 * CIA World Fact Book
 * Open Layers

Video and animation
 * Wikimedia Commons
 * WatchKnow
 * NASA's Earth Observatory (lets you build your own custom animations)
 * NextVista.org (library of educational videos by and for students and teachers)
 * Internet Archive
 * Tree of Life (some content is not open licensed)

Music
 * ccMixter
 * Wikimedia Commons music
 * Internet Archive (wide variety of stuff here, not all open)
 * Partners in Rhyme
 * MusOpen

Spoken word
 * Spoken Wikipedia
 * Library of Congress
 * Voice of America
 * Internet Archive (wide variety of stuff here, not all copyleft)

Sound Effects
 * The Freesound Project
 * Partners in Rhyme
 * SoundBible.com Free Sound Clips
 * US Fish and Wildlife Service (animal sounds)

Search engines for other copyleft content
 * Creative Commons
 * Wikimedia Commons
 * Common Content
 * ibiblio

Ebook sites

Educational open content sites