UnSplash (great photography)
http://unsplash.com/

Atlantic Monthly's Survey of Free Image Sources (new)
http://www.theatlantic.com/technology/archive/2013/08/a-guide-to-the-web...

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

Wikimedia Commons
http://commons.wikimedia.org/wiki/Main_Page

Music
Dan-o
http://www.danosongs.com/#music

Flikr Creative Commons
http://www.flickr.com/creativecommons/

Books
Project Gutenberg
http://www.gutenberg.org/