PhysBeans is a free toolkit which simplifies the construction of applets for physical simulations. It contains a set of building blocks which allow to construct an applet in a graphical way. It can be used together with any Java IDE that supports Java Beans.
The PhysBeans library is described in the book cliXX PhysBeans (in German). The library has been used for the applets on the supplemental CD-ROM of the Taschenbuch der Physik (ed. H. Stöcker), for a course on nonlinear and chaotic systems and general lectures on physics for engineers.
If you have problems with the applets or the library, are interested in more details or want to contribute to the project, feel free to contact me.