Some web sites are allowing their users to choose a Creative Commons license. This nonprofit organization offers a number of licenses which users can designate their own terms ranging form 'Some Rights Reserved' to public domain. Creative Commons-licensed materials are not all public domain, you will have to look closely to be sure. CC attributions vary, therefore you must read the summary of the license to understand how to use the image, texts, videos, music etc.
For instance, Tufts University Open Courseware uses the Attribution-Noncommercial-Share Alike 3.0 United States Creative Commons license, which means if you use this as a reference, you must specify Tufts as the author as well as share what you create from their work in a similar way. This is not public domain but allows users to access content, adapt, and share it as well.