Mozilla has released Prisim last week (previously known as WebRunner).

So what is Prisim?

“…a simple XULRunner based browser that hosts web applications without the normal web browser user interface.”

Basically it is a dedicated website in a desktop client window that lets the website basically act like a desktop client application.   So far it looks pretty slick for such a early release.

Windows download: http://starkravingfinkle.org/projects/webrunner/prism-0.8-win32.exe