The following websites contain thousands of openly licensed resources that can be freely used with proper citation. Speak with your librarian liaison about more specific resources that may be available in your subject area and see the Adelphi Libraries LibGuide on Open Educational Resources.
They work with a national network of partners to make millions of materials from libraries, archives, museums, and other cultural institutions across the country available to all in a one-stop discovery experience
HathiTrust: Founded in 2008, HathiTrust is a not-for-profit collaborative of academic and research libraries preserving 17+ million digitized items. HathiTrust offers reading access to the fullest extent allowable by U.S. copyright law, computational access to the entire corpus for scholarly research, and other emerging services based on the combined collection.
Internet Archive: Internet Archive is a non-profit library of millions of free books, movies, software, music, websites, and more. They also maintain The Wayback Machine which seeks to preserve the history of the Internet.