Links

 
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