CodePen - A front end web development playground.
CodeShare.io - Realtime code sharing for developers
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
JS Bin - Sample of the bin:
Visual Studio Live Share - Real-time collaborative development