JSFiddle is a lightweight cloud JS editor designed for developers who want to tweak and test blocks of JS, CSS and HTML code.
It's not as full featured as the other options (and technically isn't really an IDE) but is very useful for editing smaller blocks of code in the cloud.
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Pro CoffeeScript and SCSS support
Pro Allows collaborating on code
JSFiddle makes it easy to save the code snippets and send the link to others who can view and edit the code.
Pro Supports a wide variety of frameworks and extensions
JSFiddle can switch to selection of frameworks and extensions including jQuery, AngularJS, ReactiveJS, D3 simply from a dropdown.
Con Not actually an IDE
JSFiddle is not actually an IDE and is only suitable for small blocks of code.