Secondary Science
PhET Interactive Simulations, a project at the University of Colorado Boulder, is a non-profit open educational resource project that creates and hosts explorable explanations. Students are allowed to change variables in a safe environment to see what happens. This includes skateboarding, circuits, states of matter and pretty much everything else under the science umbrella. There are activities and questions at different levels for each simulation.
Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom.
The Physics Classroom is intended for beginning physics students and their teachers. It offers a tutorial, interactives, concept builders, Shockwave studios, multimedia studios, and much more.