- Plunker – http://plnkr.co/
- JSFiddle – http://jsfiddle.net/
- JSConsole – http://jsbin.com/
- Codepen – http://codepen.io/
Of these, JSFiddle seems to be used the most.
An additional site mentioned is cdnjs (http://cdnjs.com/), which hosts many, many more script libraries than Google or Microsoft’s CDN, from which you can reference a library in one of the above tools.