water is a live coding sketchpad. Code modifications are instantly displayed - no need to refresh your browser. Click on a number, adjust its value via the popup slider, and watch your work change on the fly!
100% totally based on Bret Victor's Inventing on Principle talk, which is one of the best talks I've ever seen. If you watch only one talk this year, make sure it's this one.
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 and start coding!