Here are three of my favorites mentioned:
This site was one of the first science museums to build a site on the World Wide Web. This site now contains over 15,000 Web pages exploring hundreds of different topics.
Science NetLinks (K-12)
The Thinkfinity partnership provides free, Internet-based content across academic disciplines. Science NetLinks’ role is to provide a wealth of standards-aligned resources for K-12 science educators, including lesson plans, interactive and reviewed Internet resources. Site features include Interactive Lessons, Resources, Benchmarks and Science Updates.
PhET (Physics Education Technology) (9-12)
This site from the University of Colorado has fun, interactive simulation of physical phenomena that make bridges to the real world. A collection of 40 Java applications for introductory physics is available.