Creative Commons
http://search.creativecommons.org/
iStockphoto.com
http://iStockphoto.com
Wikipedia: Public domain image resources
http://en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources
Music
Dan-o
http://www.danosongs.com/#music
Animoto
http://animoto.com
Books
Project Gutenberg
http://www.gutenberg.org/