Panda Press R.E.A.L. Science Odyssey encourages students to Read, Explore, Absorb, and Learn science. 

Khan Academy has many science based videos for high school students.

Cell Craft is a great game to teach how cells work by starting from scratch and adding parts to make a fully functioning cell. 

PBS Kids is a great place for young children to play games and learn more about the natural world. 

Westfield Studios Biology 101 is a Biology curriculum for high school.

Science for High School/Science in Your Home offers text books and kits to help you teach a Physical Science, Chemistry, and Biology.