Online Scriptable CAD!
Actually it turns out that somebody beat me to it! See http://joostn.github.com/OpenJsCad/ - it's a lot better than my quick hack!
Drag in the window above to change the view, and edit the code below to change the model. A browser with WebGL is required to see the results. See the csg.js Documentation for the available commands.
Download STL (You must change the extension to '.stl')