PHP DEK provides a package through which you can create web-based desktop applications. Such applications can be based on unlimited windows and the end user won't be bothered to refresh and reload the page content over and over again.
Web-based backends, HTML5 games or chat applications could be great examples.
The package provides abstract model for creating any kind of windows plus some classes covering the basic operations for displaying, creating or modifying data.
The package is fully customizable and your desktop can look like Win7, Windows Vista, Gnome, KDE, etc.