The Public Domain encompasses:
Public domain is different than "publicly accessible" or "free online." Read more about the Public Domain here.
Everything published by the US Federal Government is free of any copyright restrictions. The research, reports and websites can be useful resources for your courses.
Here are some examples of US Federal Databases:
(Flash - may not be viewable on all devices) Slide the red arrow down to the date of publication and read the information in the boxes on the left.
" Public Domain Slider" by Michael Brewer and ALA Office for Information Technology Policy is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License