I made a website with some of the same goals in mind that takes it quite a bit further than this. That being said, I can't even come close to pretending its less than 56 lines of code.
Check out: http://plunker.no.de
Some neat features:
* Live collaboration using 'streams'
* Plunks run as first-class web citizens letting AJAX and company work out of the box
* Syntax hilighting using ajax.org's amazing ace editor
* jsHint
* Live compilation/rendering of coffee-script/markdown and I forget what else
The full source of the running app is at: https://github.com/ggoodman/stsh
Also a ground-up rewrite on http://plnkr.co has its code on: https://github.com/filearts/plunker