I open sourced my "toy" version of JavaScript physics engine just few days ago too. It's not really for serious work, but may be (hopefully) simple and good enough for beginning programmers to learn one way of starting such a project.
<a href="https://github.com/fuzzthink/SPE.js" rel="nofollow">https://github.com/fuzzthink/SPE.js</a>
All the physics engines in JS are documented on the very valuable JSWiki: <a href="https://github.com/bebraw/jswiki/wiki/Physics-libraries" rel="nofollow">https://github.com/bebraw/jswiki/wiki/Physics-libraries</a>
The performance of the demo is awesome. Has anyone tried to do anything more complicated and tested performance. Would be fun to make a js game from this.