Core Knowledge Units
TeacherLink Free Lesson Plans
Yale New Haven Teachers
Social Studies Free Lesson Plans - Ancient, American, World History
Utah Education Network
BBC Education
Huge Number of Lesson Plans organized by subject (BC)
Smithsonian Lesson Plan Base
Multi-Subject Lesson Plan Site (Ed Sass)
Social Studies Lesson Plans (mrdonn)
Holiday Lesson Plans & Units (mrdonn)
Pro-Teacher
Awesome Library
AtoZ TeacherStuff (lessons, themes, more)