Many of them have also been translated into many languages, and are open source so they can be modified if you want.
I have, of course, learned to actually investigate such claims, although this is only, only an issue when universities and foundations purport to be providing open source software to K-12. For all intents and purposes, the entire rest of the software industry gets this right (ok, maybe not embedded system vendors). Universities and foundations get it wrong, and continue to hold out "open source" software to educators like Lucy tempting Charlie Brown with a football.
Anyhow, here's the licensing page:
What does this mean?
PhET Interactive Simulations by The PhET Team, University of Colorado are licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.
The interactive simulations developed by The PhET Team may be freely used and/or redistributed by third parties (e.g. students, educators, school districts, museums, etc.) as long as that use or distribution does not involve commercial uses (e.g. reselling the simulations, distributing the simulations through a website that makes money off of ads, etc.). If you are interested in commercial uses, see next section.
For commercial use and distribution of sims:If you are interested in alternative license options, please contact PhET at firstname.lastname@example.org.
Source code for sims:
The PhET sourcecode is licensed under a Creative Commons GNU General Public License.
The problem here is very basic. They're trying to use a non-commercial license and the GPL, which allows commercial redistribution. It doesn't make any sense. This is the mess the Hewlett Foundation, NSF (i.e., you, American citizen), and King Saud University get for their money. Pathetic.