JSFiddle has some options that you can set, such as choosing Mootools or jQuery. It also lets you tidy up the appearance with your code with the click of a button. This doesn’t seem to change your code at all, but can add line breaks and indentations.
When you’re done with your code, you can share a link to it, share it on Facebook or Twitter, or even embed it on a page. That makes JSFiddle not just great for testing out some code, but having someone else take a look at it for you.
Do you have any tools that you use when trying out code?