Live coding sketchpad for d3 visualizations written in Coffeescript. Code modifications are instantly displayed - no need to refresh your browser.
Based on Gabriel Florit's Water live coding sketchpad (original water)
The beautiful code editor, Ace, is an open source project by Ajax.org.
Any static file web server will work - for example,
python -m SimpleHTTPServer 8888
Then hit http://localhost:8888/water.html#bars and start coding!