Simple Mechanics Simulator (SiMS) is an open source physics simulation library I wrote in Scala. It's main features include:

    * 2D rigid body simulation
    * Simulation of joints
    * Collision detection and response

Important aspects of a developer's point of view are the extensibility and - as the name says - simplicity of the simulator. Unlike many other simulation libraries, SiMS is structured in a way that makes it easy to enhance or add new features to the library.
For example, a developer can implement a better collision detection system or create new types of shapes by simply inheriting from classes.
Moreover, SiMS is - thanks to Scala - programmed in a very concise manner, thus making the library very simple to understand.
The library was created under the terms of a one-year project at school. Having to comply with the evaluation criteria of a project in german language, I had no choice but to do part of the project in that language. Thus, all comments in the code and a tutorial on the simulator are written in german. However, the source code is in english; that is class, variable and method names are all in english. The comments may be translated in the future.
All source code is available at http://sourceforge.net/projects/simplemechanics
The site also provides a small GUI program, "graphyx",  for testing and visualizing SiMS.

If you want to give it a try, I'd be very interested in your opinions and suggestions,
Jakob Odersky