We have featured JS Bin 2 days ago. And our lovely readers have pointed out MooShell, which helps us writing and testing HTML, CSS and Javascript too. You can easily debug your Mootools code online with MooShell. Just load the page, edit relevant fields and load the result into an iframe. Also, it does allow to save results for further usage like this example.

However, MooShell is still in an early stage of development and definitely lacks some features. Plans are to implement logging, choice of javascript libraries (currently works with MooTools 1.2.3 only), and more.

Requirements: -
Demo: http://mooshell.net/
License: License Free