"PhET provides fun, interactive, research-based simulations of physical phenomena for free.
To help students visually comprehend concepts, PhET simulations animate what is invisible to the eye through the use of graphics and intuitive controls such as click-and-drag manipulation, sliders and radio buttons. In order to further encourage quantitative exploration, the simulations also offer measurement instruments including rulers, stop-watches, voltmeters and thermometers. As the user manipulates these interactive tools, responses are immediately animated thus effectively illustrating cause-and-effect relationships as well as multiple linked representations."
Why UDL? PhET simulations offers multiple means of representation and action and multiple means of engagement. There is also a translation tool to translate any of these simulations in a multitude of languages.