Online Textbook: Choose textbook sites, middle school book, click on appropriate class book http://www.phschool.com/math
National Geographic for Kids http://www.nationalgeographic.com/kids/
Time Magazine for KIDS http://www.timeforkids.com/TFK/teachers/wr/0,27955,080502,00.html
Balloon Invaders (Multiplication and Division) http://www.mathplayground.com/balloon_invaders.html
Space Racer X (Multiplication) http://www.mathplayground.com/spaceracer_multiplication.html
Math Playground: Fun and interactive games http://www.mathplayground.com/games.html
6th Grade Math Practice: This link will take you to the measurement section but if you scroll up or down you will view all the 6th grade concepts. http://www.internet4classrooms.com/skills_6th_math.htm#measurement
NASA for middle-school students http://www.nasa.gov/audience/forstudents/5-8/features/index.html