McDougal Littell, a school textbook publisher, has some great resources available for teachers and students in many different areas. I'm going to focus on the science resources, but you can check out other subjects from this site too.
The starting site, is Class Zone. From here, you select the subject (Language Arts, Math, Science, Social Studies, World Languages), grade level (middle vs. high school) and your state. You can pick any state and you will get some different sites based on what state you pick. These resources are free. There are some resources, like a teacher center and online textbook, that are password protected.
From there, you are brought to a list of textbooks. Click on the one that has your topic, and you will be brought to a site with a huge number of resources.
For example, selecting "science" and "high school" and "CT" gets you a selection for Biology, Chemistry, and Earth Science.
Under Biology, there are a huge variety of resources. Animations that explain difficult concepts, virtual labs, virtual dissections, Interactive review with a variety of activities, quizzes, webquests, and links to other resources.
Each subject area has different resources, but all of them are excellent. Check it out for your subject.
(PS - this blog post was completely created, including links and screen grabs, on my Google CR-48, proving that Chrome OS (and other OS's like HP webOS) are very useful and powerful.)