Open Textbooks and Materials for Open Source Education: Classroom Aid http://classroom-aid.com/open-educational-resources/curriculum/ Portal.
LibreTexts https://chem.libretexts.org/ Note other subject tabs at top of screen.
BCampus Open Textbook Summit https://www.cccoer.org/2017/06/07/bccampus-open-textbook-summit-highlights/ This is included because it alerts you to a Summit developed and sponsored by Community College Consortium for Open Educational Resources https://www.cccoer.org/ which is working to develop/locate more open resources at the community college level.
Open Culture: The Best Free Cultural and Educational Media on the Web http://www.openculture.com/ and Page 2 http://www.openculture.com/page/2 Note list of links on right of screen. Portal.
TeachThought: 4 Sources of Open Source Textbooks http://teachthought.com/technology/5-sources-of-open-source-textbooks/ Scroll down to last paragraph for the list and links.
College Open Textbooks http://www.collegeopentextbooks.org/ Click on Textbook Listings tab on left.
Open Textbook Library http://open.umn.edu/opentextbooks/
Open AccessTextbooks: Websites http://www.openaccesstextbooks.org/websites.html Portal.