TeachersNet (an all-in-one site for teachers: lesson plans, discussion groups, mentoring, career opportunities and MUCH more!)
http://www.teachers.net/

Global Schoolhouse Projects and Programs
http://www.gsn.org/project/index.html

The Education Place (K-8 on-line teacher resources)
http://www.eduplace.com/

Volcano World! (comprehensive site dedicated to everything related to volcanoes with lesson plans, classroom activities and projects, photos, movies, and how to make a volcano model with lava)
http://volcano.und.nodak.edu/

The National Archives and Records Administration (view the exhibit hall and documents on-line with amazing clarity and detail using special imagery photography)
http://www.nara.gov/

Today in History (features a detailed daily historical event from the Library of Congress-National Digital Library)
http://memory.loc.gov/ammem/today/today.html