PhET: Free online physics, chemistry, biology, earth science and math simulations

PhET provides fun, free, interactive, research-based science and mathematics simulations. We extensively test and evaluate each simulation to ensure educational effectiveness. These tests include student interviews and observation of simulation use in classrooms. The simulations are written in HTML5 (with some legacy simulations in Java or Flash), and can be run online or downloaded to your computer. All simulations are open source (see our source code). Multiple sponsors support the PhET project, enabling these resources to be free to all students and teachers.
More About PhET's Design
To help students engage in science and mathematics through inquiry, PhET simulations are developed using the following design principles:


Encourage scientific inquiry
Provide interactivity
Make the invisible visible
Show visual mental models
Include multiple representations (e.g., object motion, graphs, numbers, etc.)
Use real-world connections
Give users implicit guidance (e.g., by limiting controls) in productive exploration
Create a simulation that can be flexibly used in many educational situations
Several tools in the simulations provide an interactive experience:


Click and drag to interact with simulation features
Use sliders to increase and decrease parameters
Choose between options with radio buttons
Make measurements in your experiments with various instruments – rulers, stop-watches, voltmeters, and thermometers.
As users interact with these tools, they get immediate feedback about the effect of the changes they made. This allows them to investigate cause-and-effect relationships and answer scientific questions through exploration of the simulation. For more information, visit our research page.