[An (automated) port of Box2dflash 2.0][link], which is a bit slower than the older (and dead) [Box2D 1.4 JS port][14], but is less complicated to use and has fewer dependencies.
[It has a Github, too.][git]
[link]: http://29a.ch/2010/4/17/box2d-2-flash-ported-javascript “Box2Dflash Version 2.0 ported Javascript — 29a.ch – Jonas Wagner’s Blog”
[git]: http://github.com/jwagner/box2d2-js “Box2d2-js on Github”
[14]: http://box2d-js.sourceforge.net/ “Box2dJS 1.4”