The following link is a list of "Best of the Best Science Websites for Elementary Grades".
Best of the Best Science Websites for Elementary Grades
It is a truly amazing collection of science websites. The link to the Bitesize Science is the updated version. In my links to interactive simulations in the Properties of Matter subpage and the Solids, Liquids, and Gases subpage, I reference the older version of the website because it has the simulations. You can link to the older version from the updated one. Both have so many engaging videos and activities!
I found the link to the Exploratorium mind boggling just because there was so much awesome content!