PhET Interactive Simulations, a project at the University of Colorado Boulder, is a non-profit open educational resource (OER) project founded in 2002 by Nobel Laureate Carl Wieman. PhET began with Wieman’s vision to improve the way science is taught and learned. Their stated mission is "To advance science and math literacy and education worldwide through free interactive simulations."
PhET provides fun, free, interactive, research-based science and mathematics simulations. These tests include student interviews and observation of simulation use in classrooms. The simulations are written in Java, Flash or HTML5, and can be run online or downloaded to your computer. All simulations are open source
Following video explains what is PhET. The PhET simulation software is available for Windows, Linux and Mac platforms. The PhET simulators for the appropriate platforms can be downloaded from here.