JSFiddle

 JSFiddle is one of the most useful websites for web design and testing. This website lets you test new code, find errors or bug, and more. It gives you a place to add Javascripts, CSS, and HTML. When your all done coding it will run it and give you the end result. When your done you can share the code by sending them a link or embed the page into a website.

Description From JSFiddle:

JsFiddle is a playground for web developers, a tool which may be used in many ways. One can use it as an online editor for snippets build from HTML, CSS and JavaScript. The code can then be shared with others, embedded on a blog, etc. Using this approach, JavaScript developers can very easily isolate bugs. We aim to support all actively developed frameworks – it helps with testing compatibility

If you would like to see some examples of jsfiddle in action click here.