NASA for middle-school students http://www.nasa.gov/audience/forstudents/5-8/features/index.html
National Geographic for Kids http://www.nationalgeographic.com/kids/
Fun activities - by girls....for girls. http://www.missoandfriends.com/
Math Activities for Grades K - 12 http://nlvm.usu.edu
Helps improve math skills interactively. It has a game room, worksheets, and flashcards. http://www.aplusmath.com/
Helps with understanding through practice, review, tutorials, and games. http://www.ricksmath.com/
Helps with practicing fractions visually and solving operations. http://www.visualfractions.com/
An animated, interactive dictionary for students explaining over 500 common mathematical terms. http://www.amathsdictionaryforkids.com/
Math games at different levels of difficulty. http://www.funbrain.com
Arithmetic exercises to illustrate basic math lessons. http://www.aaamath.com