OpenEd: www.opened.io with more than 250,000 videos, games, and other resources aligned with standards.
The Digital Public Library of America: http://dp.la and the Library of Congress www.loc.gov have copious primary-source materials – manuscripts, artworks, maps, and sound recordings.
Gooru: www.goorulearning.org is an open and collaborative online community used by more than 400,000 teachers in 140 countries and all 50 states.
Creative Commons: http://search.creativecommons.org is an effective tool for finding copyright-free media.
specializes in high-quality curated STEM content and has links to more than 15,000 resources.