It is always hard to find the best "free" resources to suggest to teachers. Below you will find links to three pages where I have begun to compile these resources. Let me know if there are any I should add!
Photos, Images, and Clipart http://katiechristo.pbwiki.com/Photos,+Images,+and+Clip+Art
Primary Sources http://katiechristo.pbwiki.com/Primary+Sources
Sounds and Audio http://katiechristo.pbwiki.com/Sounds+and+Audio