These sites are good places to start looking for works in the public domain; however, you should always check copyright notices for the specific work before using it.
USDA Flicker Page (Nearly 52,000 images!)
Project Gutenberg (free eBooks)
Click here to submit a free resource you use to help out our K-12 community!
Here are the most recent submissions:
https://creativecommons.org You can search for public domain resources from this site.