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 various types of learning resources by grade level and type

Smithsonian Institution - resources and learning labs for K-12

Powerful Geography Lesson Plans and Units  - has several lesson plans and units for grades K-12

National Geographic - free maps, videos, interactives, and other resources for K-12

BBC - lessons for middle and high school age students

BBC - for K-8 students