OS.js is a JavaScript web desktop implementation for your browser with a fully-fledged window manager, Application APIs, GUI toolkits and filesystem abstraction.
Free and open-source
Simple installation and setup
Fully documented
Works in any modern browser
Fully scalable UI
Can be deployed on any machine running node
Also comes with an alternative PHP server
Customizable and extendable
Supports multiple types of Authentication modules
Virtual Filesystem that allows you to connect to a range of endpoints
Desktop and Window Manager built to feel like the real deal
Session support so you can save open application states
Localization and translation support
Supports Windows Live API, Google API
Supports Google Drive, Microsoft OneDrive and Dropbox