After seeing it featured in several articles as a tool for live examples, I can only say that JSFiddle is a dream come true: an experimental environment where you can test your HTML, JavaScript and CSS and see the results right there. You can even choose a framework to work with (jQuery, Mootools, Prototype) and start coding away.
Highly recommended. I’ll definitely be using it to test during development!
Get your hands on it: http://jsfiddle.net/