Single Code Base : Deploy on the Web and in Windows...
As you can tell I really love this VisualWebGui stuff from Guy Peled. Take a look at the latest ability, its called "Dual Mode". Essentially you build your web site using their technology and you deploy to the Web normally, or deploy as a complete Windows Forms application! Of course you will have to make special consideration for your business objects and more importantly your data source but its just damn cool.
I know I have been waiting for something like this for many, many years now.