CoffeeScript console in the Web Inspector.
The extension keeps everything on local storage so there is no server dependency and a live Internet connection is not required. Everything happens in your browser! CoffeePad is built with Koding's KD Framework and uses CodeMirror as the editor and CoffeeScript's browser compiler as the backend compilation engine.
CoffeePad is open source and its source code is available here: https://github.com/gokmen/coffeepad