Here are just a few websites I think are some of the most beneficial for a classroom, or a teacher.

http://www.brainpop.com/educators/lesson_plans/

Awesome website for teachers - many, many lesson plans.

http://www.learn4good.com/kids/

Another great site for students and teachers.

http://www.smithsonianeducation.org/students/index.html

This one is really cool for students.