Extreme Science - This site has a variety of articles, some with videos to go with them, on a many different science topics. Most of the articles deal with extremes in science, as the name of the site suggests. This is one that my daughter will enjoy and my son, who struggles with reading, will appreciate the video component.
Popular Science - This site is best suited for older students, but is full of articles, photo galleries and videos about a variety of technological science.
The Rudiments of Wisdom Encyclopedia - This is the link to the Science section of The Rudiments of Wisdom Encylopedia. Here there is an alphabetical list of science subjects, which, when clicked on will take you to an explanation of the subject in cartoon form.
The Science Explorer - Fun science experiments and activities that can be done with things found around the house.
ScienceMonster - While this site is still under construction there are some good resources available here, with more on the way.
Sea and Sky - Exploring the Splendors of the Sea and Wonders of the Universe.
Exploratorium - The musuem of science, art and human perception. There is lots and lots of cool stuff here.
Science News for Kids - Science News for Kids helps kids keep up with current scientific discoveries and theories.
National Geographic for Kids - National Geographic for Kids is one of my very favorite learning sites. There are so many resources here, fun games, videos, great photos, and all presented in a way that holds a child's attention.