Teacher.org contains lesson plans for grades K-12 on various subjects. There are tons to choose from and teachers are encouraged to share their lesson plans.
Kids.gov is a great resource for activities, worksheets, and lesson plans on a variety of subjects.
Kids in the House provides resources, activities, and lesson plans regarding the U.S. House of Representatives.
The United States Mint contains resources, activities, and lesson plans on the history of U.S. currency.
Library of Congress is an excellent resource for English teachers. It provides resources, activities, and lesson plans on classical literature.
Smithsonian Education provides lesson plans on various subjects. This website also has access to over 2,000 resources.
National Archives allows educators to search through a variety of lesson plans and create their own.
Ben’s Guide contains lesson plans and resources on the U.S. Government and Ben Franklin.