Creative Commons - http://search.creativecommons.org/ - Aggregate search on different commons sites for different media from images to sounds and videos. Media are opensourced and free to use. Some if Attributed.
Freely Available Images and Multimedia for Educational Use: By John Hopkins University http://www.cer.jhu.edu/mediaresources.html
Top 5 Sites to Find Images for Student Use: By Edublogs - http://edublogs.org/2010/09/17/edublogs-weekly-review-top-5-sites-to-find-images-for-student-use/
The Best Online Sources for images by Larry Ferlazzo - http://larryferlazzo.edublogs.org/2008/10/11/the-best-online-sources-for-images/