Scholastic http://teacher.scholastic.com/activities/CLF/index.htm
FunSchool http://funschool.kaboose.com/index.html
First in Math www.firstinmath.com
FunBrain http://www.funbrain.com/
Multiplication www.multiplication.com
Try Science! www.tryscience.org
National Geographic Kids http://kids.nationalgeographic.com/kids/
Time for Kids http://www.timeforkids.com/