https://en.wikipedia.org/wiki/Online_source_code_playgrounds#Online_Web_Client-Side_Source_Code_Playgrounds *
codepen is pretty freakin' cool! http://codepen.io/pen/
for example, they include http://blog.codepen.io/2015/02/23/code-folding-settings-customize-editor/
and also a variety of visual options http://blog.codepen.io/2015/02/17/prefer-tabs-prefer-spaces-prefer-specify-indentation-width-got-covered/
... and they even have their own "frontend jobs" board! http://codepen.io/jobs/ take that, "indeed"! ;)
there's a bunch of some-quite-impressive "demos" listed throughout their Twitter @ https://twitter.com/codepen
for example http://codepen.io/kirkbyo/full/qEYNKm/
and this "KeepCalmAndPortalOn" scroller... http://codepen.io/arti360/full/myLEbw
* list...
CodePen
CSSDesk
JS Bin
JSFiddle
JQ.VER.SION
Khan Academy
W3Schools
WebFiddle