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/
Save the Manatees http://www.savethemanatee.com
Earth and Sky (great for nighttime sky watching) http://www.earthsky.org