Worlds of math & physics is a site (and a blog) dedicated to some interesting mathematics and physics topics.
There will also be interactive simulations, made with Wolfram Mathematica, that can be interactively used with the free CDF Player.
The first time you open a page embedding a CDF (Computable Document Format) simulation 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).