OpenJsCad
Enthusiastic about OpenSCAD but somewhat frustrated by the limitations of its language, here is an alternative using Javascript. It's free, completely open source, still seriously under construction, based on Evan Wallace's CSG.js library and can be used from within a WebGL supporting browser. Currently Chrome and Firefox work well.
To use OpenJsCad 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. It has built in support for 2D and 3D math, and handy features such as Connectors and Properties which make it possible to create more complex models.
Contribute
Contributions are welcome, fork me on GitHub!
The only content here is in the gh-pages branch at https://github.com/joostn/OpenJsCad/tree/gh-pages
The actual CSG engine is in csg.js, while OpenJsCad.js contains the user interface.
License
Copyright (c) 2012 Joost Nieuwenhuijse (joost@newhouse.nl), under the MIT license. Copyright (c) 2011 Evan Wallace (http://madebyevan.com/), under the MIT license.