National Council for the Social Studies - lessons, essays, videos and primary sources resources for grades K-12 can be accessed by members

PBS Learning Media - provides  resources by grade level and type - Elementary Social Studies U.S.  History  World History

Smithsonian Institution - resources and learning labs for K-12

National Archives - Documents and lesson ideas including resources for National History Day.

Ohio State University - a searchable site with free resources

Maine Historical Society - Maine Memory Network - includes primary sources, virtual textbook on Maine History, and Lesson Plans.

Gilder Lehrman Institute of American History - lesson plans and resources that can be searched by topic and grade level

Echoes and Reflections - lessons on the Holocaust for middle and high School students

National Council for History Education has a collection of resources including History’s Habits of Mind Resources, and webinars, blog posts, as well as teacher-vetted selections from external organizations. Every resource is free to access and requires no login.

Maine Maritime Museum - includes primary resources such as ship logs, sea journals and more, as well as lesson plans. 

OER Project - Classroom ready history curriculum. Search for lessons, activities, and more.