PhET is one of my go-to resources for teaching Science. There are dozens of science simulations in Physics, Chemistry, Math and Biology that are research based and designed to demonstrate and simulate specific phenomena. While some have a very narrow focus, others are others that are more open ended. The electric circuit simulator for instance, is sufficient to act as the focus for electricity unit of my grade nine Science class – it can even load and save files, so my students can create circuits and send them to me. For those who have not explored it, go there now.