PhET: Interactive Simulations - A site from the University of Colorado that has fun science applets about all kinds of stuff. I use this in class for various topics.
The Mechanical Universe...and Beyond - This site has streaming video of a classic science series. the videos cover discussions on various topics. You may have to register on the site.
The World of Chemistry - Another set of streaming videos about various topics, this time focused on chemistry.
Physics Applets - More applets that visualize various concepts. I use one of these for our sound unit.
Visual Quantum Mechanics - A site that has applets designed to visualize Quantum Mechanical ideas. Suited best to AP Chemistry students.
Physics 2000 - Another site with quantum mechanics topics.
General Chemistry Digital Lecture Series - Lecture series put on by Jackson Community College in Michigan. This is a good place if you want another discussion on chemistry material.
Water Quality Activities - Why is this here? I am interested in water science, but sadly don't get enough time to do it in school. This is a resource for some activities and other information.
AP Chemistry Course Homepage - Homepage for the AP Chemistry course. If you are in AP, you should check this out.