myphysicslab is a project site from  Erik Neumann, full of physics experiments you can try directly online or (better, faster) offline, simply downloading all the things in a single zip file.
Forces, vectors, springs, object collisions, and much more! you can easily control parameters and graph things, change colors, number of objects, gravity and so.

Here below the screencast of an “offline” demo

An exaustive  online documentation is available; all the project is also on github, so you can contribute and/or fork the entire project.

The project is opensource under the Apache 2.0 License.