Some of the demonstrations presented in this site were made with the software Wolfram Mathematica, and exported in the .cdf format so that they can be interactively used with the free CDF Player.
2017: see the note here-below for an important update about the functionality of CDF Player as a browser plugin.
The first time you open a page embedding a CDF (Computable Document Format) demonstration you’ll be asked to download the free CDF Player at this page (it’s not mandatory to fill the user’s description and email address fields).
After the installation of the CDF Player on your PC you’ll be able to interact with all inputs, sliders and controls of the simulation directly on your browser and to manipulate 2D and 3D graphic. In case of problems in running the CDF Player plugin with some browser (i.e. Internet Explorer 11) see this link.
Alternatively you can download the “.cdf” file (if made available) and play with it off-line through the CDF Player standalone program.
Presently CDF Player works on Windows, Linux and MAC platforms, but not on iOS and Android devices (hopefully it will be extended to them in the near future).
UPDATE (31st march 2017)
It seems that it’s no more possible to use, trough the browser, the Wolfram CDF Player plugin in the recent versions of the major browsers . So it’s advisable to download the “.cdf” file (if made available) and play with it off-line through the CDF Player standalone program.
Temporarily, if you prefer using the browser plugin, you can install Firefox ESR, that should keep alive the compatibility with the CDF Player until early 2018.