CodePen - A front end web development playground.
Edabit - Learn to Code in 90 Days or Less
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
Liveweave - Liveweave is a HTML5, CSS3 & JavaScript playground for web designers and developers
CSSDeck - Collection of Awesome CSS and JS Creations to help out frontend developers and designers.
JS Bin - Sample of the bin: