Open Educational Resources, a site that offers thousands of lessons in a variety of areas and for all grade levels.
http://www.oercommons.org/

This offshoot of cable TV’s Discovery Channel hosts hundreds of K-12 lesson plans, all written by teachers.
http://www.discoveryeducation.com/teachers/free-lesson plans/index.cfm?campaign=flyout_teachers_lesson

Privately owned and supported by advertising, this site has 3000-plus lesson plans available.
http://www.lessonplanspage.com

The online arm of the well-known magazine offers thousands of lesson plans, including many maps and other extras.
http://education.nationalgeographic.com/education/teaching-resources/?ar_a=1&ar_r=1

This branch of the nonprofit Public Broadcasting Service hosts more than 3000 free lesson plans and activities.
http://www.pbs.org/teachersource