Launch your Chrome browser and go to: http://joostn.github.com/OpenJsCad/
The benefits over openSCAD are mainly in the language itself: you can use dynamic arrays for example, and solids can be stored in variables.
Contributions are welcome, fork me on GitHub!
The only content here is the 'GUI' part (i.e. index.html) in the gh-pages branch at https://github.com/joostn/OpenJsCad/tree/gh-pages
The actual CSG engine is in csg.js. Don't fork the copy of csg.js here, but instead create a fork of this branch: https://github.com/joostn/csg.js
Copyright (c) 2012 Joost Nieuwenhuijse (email@example.com), under the MIT license. Copyright (c) 2011 Evan Wallace (http://madebyevan.com/), under the MIT license.