CodePen has an impressive amount of support for preprocessors (such as Jade, Haml, Slim, Sass, Less, Stylus, Coffeescript and PostCSS). There is also plenty of frameworks and libraries to pick from (Foundation, Bootstrap, Angular, D3, Backbone, Ember etc.).
The codepen profile page allows you to display all of your public pens, and control which order you want them to be viewed in. This is great for showing off your work to possible employers, other devs etc.To fork a pen only requires clicking one button, and you'll be able to modify the pen on your own account.You can search through other pens, either by keyword or popular, picked or recent.CodePen are awesome. This is a must have tool for any front end developer as it allows you to test out concpets and ideas before pushing them to a live enviroment.