EdTech Teacher Science - a wealth of resources broken down by subject
PHet - virtual labs and simulations, with lesson plans, to use with students
The Biology Corner - lesson plans, activities, and other resources. Mainly Biology, but also has AP and Physics resources
Discovery Education - Science Resources - Great free resources from Discovery
Understanding Science - Excellent resources that explain complex concepts in an easy to understand fashion
The Physics Classroom - Large collection of tutorials, activities, and simulations