Yep, JsFiddle is brilliant. There are a few others too. http://jsbin.com - Not as many features as JsFiddle and the preview is on a separate page. The HTML/CSS/JS isn't kept completely apart either. http://cssdesk.com - Primarily for CSS/HTML. Preview is automatically updated. http://mootools.net/shell/ - Similar but more Mootools-centric; fewer features etc than JsFiddle.
http://jsfiddle.net/amadeus/5y5nE/show/
Also, it's recommended to use jsFiddle over Mootools Shell, since MooShell is essentially a really old version of jsFiddle, and will eventually be removed from mootools.net