This looks suspiciously like Ian Storm Taylor's socrates.io: http://socrates.io
I was thinking of http://socrates.io as well. Very similar.
Ian says it's done by a friend of his.
My buddy Ian Johnson has a similar project I occasionally help out with called Tributary which is similar, but mainly intended towards data visualization development with SVG / D3.js ( http://tributary.io/tributary/2958568/ )
Gabriel Florit has a similar project called Livecoding.io ( http://livecoding.io/3419309 )
There's also the beautiful http://dabblet.com/
For times when I'm writing code that isn't necessarily frontend, I use the LiveReload OS X app: http://livereload.com/