Fellows & Friends,
The Learning Page has a really nice guide to other websites about education and social studies <http://memory.loc.gov/ammem/ndlpedu/websites.html>.
But Cindy Stout is looking for one on science... And I'll bet others out there are looking for good links to all kinds of topics taught in your schools (or even for your personal interest).
You might try the Argus Clearinghouse <http://www.argus.com>. It is a guide to Internet guides. The guides are arranged in a hierarchy and can also be searched by subject. Guides are reviewed periodically and rated with a system that is described on the site. When the web was young, this Clearinghouse was housed by the University of Michigan. Information about the company, Argus Associates, Inc. (by the company) is here: <http://argus-inc.com/>.
Back to those science sites...
NASA has a very large site <http://www.nasa.gov/> with educational resources here: <http://education.nasa.gov/>.
Education & Outreach
Many of the individual museums have education-related materials on their sites.
Like the Smithsonian, most Federal Government agencies have some educational component on their sites. For links to them, try this LC guide: <http://lcweb.loc.gov/global/explore.html>.