A small (~10) collection of web-based science activities. Teachers like the site, and I think kids do too. The activities use an inquiry-based approach with high-interest topics to build science skills. Teacher guides are available.
A useful general science site for students. Includes student hands-on activities and a teacher area.
Introduction to Newton's Laws of Motion (of which one is force = mass * acceleration -- get it?) Noisy and overdone, but may be interesting to middle and high school students.
A role-playing virtual world science exploration for middle school students. Originally a Harvard-based research project, its current status (summer 2009) is in limbo while funding is sorted out.
"he AMSER Science Reader Monthly aims to provide educators with a useful package of information about a particular topic related to applied math and science by combining freely available articles from popular journals with curriculum, learning objects, and web sites from the AMSER portal."
The page is simple: sixty symbols that link to a physics or astronomy YouTube discussion. Simple and elegant.
"Viewing nature and technology in context", a holistic, Goethean, qualitative approach to science and nature.
Nice collection of Flash-based activities; site includes a teacher section and a section for kids. From the Howard Hughes Medical Institute.
Set up a meeting for your class to meet online with grad students and examine bugs under the university's electron microscope.
Subscription-based site. Includes exploration-type activities with virtual manipulatives plus assessments (quizzes).
Web-based Logo development tool, but also a large library of simulations where rule inputs can be controlled. Can be useful for seeing how different scientific models work.