These are some of my favorite websites for finding Public Domain content for products, articles, blog posts and more. Of course, this is just a very small sampling!
Video
Photos
www.usa.gov/Topics/Graphics.shtml
http://en.wikipedia.org/wiki/Public_domain_image_resources
http://images.google.com Advanced Search – Commercial Use
http://commons.wikimedia.org/wiki/Main_Page
Artwork
http://www.princetonol.com/groups/iad/links/clipart.html
http://beinecke.library.yale.edu/digitallibrary
Text
Music / Audio
Music (Stock)
Working with content in the Public Domain often means verifying a work’s copyright status. This in-depth video and PDF document, from “Public Domain Blueprint,” explain the entire copyright research process, step-by-step.
Limitless Blog Content (Magazine Content)
Easy Money Picture Project (Image Content)
Info Gold Extreme (Government Content)