Copyright, Fair Use, Creative Commons
Read this summary of the law and your rights.
3. Creative Commons - Public Domain
Some works are published but not copyrighted. The public domain includes works that have their copyright expired (usually 70 years after the death of the copyright holder) or are created by government employees (like NASA photos).
Some works use a creative commons license which gives other people the right to use the material.This poster summarizes the types of creative commons licenses and the icons used to show the license.
http://wiki.creativecommons.org/images/6/62/Creativecommons-informational-flyer_eng.pdf
Use this page to determine which license to use for your work
http://creativecommons.org/choose/
Finding work that is in the creative commons: http://search.creativecommons.org/
Images
http://commons.wikimedia.org/wiki/Main_Page
http://www.flickr.com/creativecommons/
http://www.usa.gov/Topics/Graphics.shtml
http://www.morguefile.com/archive
Music