101 Science is an incredible site with a huge amount of resources for science.
The site has more resources than I could count, organized by topics such as "electronics", "chemistry", "biology", "physics", "math" and "photography." There are also resources for earth science, astronomy, robotics, ethics, java applets, and much more. Each main category has sub categories also.
Each topic has scores of links and resources, including articles, web sites, and videos. Some videos are from MIT, TED and more.
If you teach science, are a student of science, or just like science, this is a must see site.
Here's a look at the main Physics page:
Each link on this page has more links. Here is the page on Gravity:
(Some sub categories have much more than this even!)
This site is just tremendous with over 20,000 links and resources.