http://jsfiddle.net/
JSFiddle is more than just a sandbox for front end coders. With JSFiddle any coder can easily segment out JavaScript, HTML and CSS into a customized environment which can be run, saved and shared – paving the way for collaboration.
The JSFiddle environment can be loaded with one of 11 different libraries with support for different versions of each. For example in jQuery you can load version edge, 1.2.6, 1.3.2, 1.4.4, 1.5.2, 1.6.1 or 1.6.2. For each version of jQuery you have the option of also loading the corresponding version of jQuery UI and jQuery Lint if available. In addition additional resources can be added to the environment using the Manage Resources section on the left.
JSFiddle can Also Account for Ajax and Custom Plugins via the Additional Resource Management.
The true power of JSFiddle is the ability to put code into an all inclusive collaborative environment. A lot of front end developers have used JSFiddle to troubleshoot their code, give examples of how code should be used and create simple proof of concepts. JSFiddle is a favorite of front end developers for sharing the code relating to their questions and answers on code discussion sites like Stack Overflow.
With the consistent support of ever changing libraries and the ability to quickly and easily share an environment JSFiddle is a great resource for any front end developer.