Math:

A+ Math (Grades 2-8)

Ask Dr. Math (Grades 2-12)

Kids' Place Mathematics from Houghton Mifflin (Grades K-3)

Math Forum: Problems of the Week (Grades 4-12)

Math Square from Education 4 Kids (Grades 1-8)

Math World (Grades 8-12)

Muliplication.com Classroom Games (Grades 3-8)

That Quiz (Grades 1-10)

 

Virtual Manipulatives (Grade K-12)

                Includes: Numbers and Operations

                                Geometry and Measurement

                                Functions and Algebra

                               Data, Statistics and Probability

What's Special About This Number?  (Grade 6-12)

  Includes links to describe the number theory associated with each number.