Silverlight is an extensible framework. You can write code for it and later don’t to rewrite it, you will able to reuse it. As we could hope, there’re some project of Silverlight libraries.
Silverlight Desktop (http://www.silverlightdesktop.net/) is a framework to create windows inside a Silverlight 2 application.
It allow us to create elements with drag and drop effects, a little security framework integrable into a web application, it contains a administration section protected by password and an application log.
The code, documentation and more stuffs are available in the official web page: http://www.silverlightdesktop.net/