Exploriments is a site that has interactive simulations that help students learn and understand math and science. Students are active and engaged, rather than passive. They get to explore different topics and ask, and answer, questions like "what-if" and find the cause and effect of things. There are learning objectives for each Exploriment and everything works together and builds on previous knowledge.
The Exploriments are organized by Physics, Chemistry and Math and then further organized by topic. The physics ones are well done and I've used a few with my students already.
The free access does not give you full access or function, but there are some great things to use. You can upgrade to the premium access if you want to.