JSLint shows you where your code has problems.
JSFiddle is an online program for running web applications in a test environment. When you go to JSFiddle.net, the first thing you see is a grid with four panes:
One for HTML
One for CSS
One for Results
Enter the appropriate type of code into any of the first three boxes and press the Run button, and the results will be displayed in the Results pane.
With JSFiddle, you can even save your fiddles and email the urls to other people to check out.
Collaborate with JSBin.
JSBin is a code-sharing site that allows you to write code while other people watch you. Whether you have exhibitionist tendencies, you’re teaching a junior developer, or you’re collaborating with other programmers on a project, the functionality in JSBin can be very helpful for working out bugs, getting feedback, and sharing code.
Make your code prettyy.
Inserting new lines
Breaking lines of chained code
Inserting spaces before conditional statements
Making indentations standard throughout the script
Point and click regular expressions.
The JSON formatter validates and arranges JSON data.
The JSON formatter and validator allows you to paste in unformatted JSON code, such as the code you would get from copying from the Chrome Developer Tools. It then makes the code pretty and makes sure that it’s valid.
JShint detects problems with your code as you write it.
Mozilla Developer Network