DPLA collects information about digitized resources from institutions all over the country so that all of their content can be searched through the single DPLA interface.
A local partner of DPLA, Digital Commonwealth provides a similar service for collections held by institutions in Massachusetts.
Digitized materials from the collections of the world's largest library, an agency of the U.S. government's legislative branch.
Most of the categories allow you to select "Free access" under the Access options on the results page (in the right margin).
