it is a solution similar to Windows Explorer or Gnome desktop in Linux. Except, it works in web-browser and is written in GWT. Web Desktop Manager allows running GWT widgets in draggable and resizable windows organized in so-called desktops. These desktops can be navigated through built-in menu.
It is one of the top 10 GWT project in official Google Gallery - http://gwtgallery.appspot.com/results?topapps=true