Educator.com: Offers hundreds of quality instructional videos (Not Free)
Curriki.org: Thousands of thoroughly vetted online learning activities in all major K-12 subject areas in many formats.
OpenStax.org: Peer Reviewed, openly licensed, 100% free learning resources for Math, Natural Science, Social Science and Humanities
CK12.org: Learning activities for K to 12
OAsis: Commonwealth of Learning’s online institutional repository for learning resources and publications