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/
Mathscore http://www.sunnyvale.mathscore.com/
Fun Brain Arcade - Math, Science and other cool games http://www.funbrain.com/
Science Monster - More science fun and math links http://www.sciencemonster.com/