NASA for high school students http://www.nasa.gov/audience/forstudents/9-12/features/index.html
National Geographic http://www.nationalgeographic.com
A free teacher developed chemistry tutorial site for high school students. http://www.ausetute.com.au/index.html
Get a free download of a desktop molar weight calculator for chemistry students. http://www.chem4free.info/mcdalton/
Free Physics tutorial organized by concept maps. It has interactive problem solvers. http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Explains anything and everything you ever wanted to know about how stuff works. http://www.howstuffworks.com/