The following is a collection of sites that provide clipart, photos, music and sound effects that are in the public domain or are licensed to allow their use under certain conditions including Creative Commons licenses.
Clipart
WPClipart
WPClipart is an ever-growing collection of artwork for school kids and others that is free of copyright concerns as well as safe from inappropriate images.
Public Domain Clip Art
A huge collection of public domain clip art.
Clker.com
The online royalty free public domain clip art
Photos
MorgueFile
Public image archive with very friendly licensing.
Pixabay.com
An alternative to expensive stock photography with friendly licensing (search results also include 'premium' results from Shutterstock).
Flickr
One of the largest online photo sharing sites. This site contains both photos and videos that are copyright protected and offered under a Creative Commons license. To make sure you are searching Creative Commons licensed photos, start at this link:
http://www.flickr.com/creativecommons/
Music and Sound FX
Free Music Arcive
The Free Music Archive offers free downloads under Creative Commons and other licenses.
ccMixter
http://ccmixter.org/find-music
ccMixter is a community music site featuring remixes licensed under Creative Commons where you can listen to, sample, mash-up, or interact with music in whatever way you want.
The Freesound Project
A collaborative database of Creative Commons licensed music and sounds.
Open Educational Resources
CK-12 Foundation
CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the “FlexBook™,” CK-12 intends to pioneer the generation and distribution of high quality educational content that will serve both as core text as well as provide an adaptive environment for learning.