The processing.js demos look interesting! None of these strike me as code editors, though. I see them more as code execution environments. To me, an editor is something more advanced then a textarea. Something like EditArea or ACE.

We have also done some work on this area related to automatic assessment. One of our papers went through various browser-based tools for assessing different aspects of JavaScript programs. The code is on GitHub. An interesting project I recently ran into is BetterExamples.js which seems to provide step-debugging functionality for JavaScript.

I’ve also played around with Skulpt (the JS-based interpreter for Python your students also used) to add some simple assessment functionality. Code is on Github. Although it is now written for Node.js to be used on server-side, it should be trivial to make it work in browser.