Tandem provides supporting material for simulations using phetioEngine.js. This library is licensed so that it can be used in published simulations (as opposed to phetioEngine.js which is licensed separately).

By PhET Interactive Simulations http://phet.colorado.edu/

The PhET Development Overview is the most complete guide to PhET Simulation Development. This guide includes how to obtain simulation code and its dependencies, notes about architecture & design, how to test and build the sims, as well as other important information.

See the license