A combination of etherpad and processing.js.
Code is run whenever the user pauses in typing.
A version should be running on http://ethasketch.com unless it is broken.
EtherPad is a web-based realtime collaborative document editor.
EtherPad used to live at http://etherpad.org, but has been acquired by Google and open sourced.
The current EtherPad repository lives at http://github.com/ether/pad
Processing.js is a javascript implenentation of processing, an open source programming language and environment for people who want to program images, animation, and interactions.