The University of Colorado Boulder has a fabulous site for interactive science simulations.
Great Java Simulations for Science Class
In one of my favorite simulations, you can create a circuit using light bulbs, wires, resistors, batteries, etc. as well as household items such as a dollar bill and a paper clip. When the circuit works, the light bulbs light…the more resistance, the dimmer the lights.
Loads of other great simulations HERE!