Adaptive Curriculum offers interactive lessons in Math and Science. Each lesson gives an explanation and then has interactive practice or an experiment. The lessons are extremely well-done, covering experiments, problem solving, skills application and dynamic modeling through real-world applications. Topics cover math and science for grades 6-12.
An annual license is $99.95 for either High School or Middle School Math or Science, $129.95 for both subjects at one level. To get both levels, it is $149.95 for either Math or Science or $199.95 for both. All of these licenses are good for up to three students. With your subscription, you get hundreds of Activity Objects (their term for their interactive lessons), a teacher guide and online assessment for each AO, printable activity sheets, an online interface to assign and track AOs and a reporting system.
Homeschool Buyers Co-op is currently offering Adaptive Curriculum at 50% to 60% off (please read their offers carefully, some of the additional content may not be included). See also Uzinggo listing, which gives you the same curriculum, but with a better interface to allow students to work independently and earn badges. And for the best value for the Middle School level, see my post about Learning.com.