I fear this is a duplicate, but I haven't found it yet.
Anyway to answer your question: use Prism
Prism is a simple XULRunner based
browser that hosts web
applications without the normal web
browser user interface. Prism is based
on a concept called Site Specific
Browsers (SSB). An SSB is an
application with an embedded browser
designed to work exclusively with a
single web application. It doesn’t
have the menus, toolbars and
accoutrements of a normal web browser.
Some people have called it a
"distraction free browser" because
none of the typical browser chrome is
used. An SSB also has a tighter
integration with the OS and desktop
than a typical web application running
through a web browser.

Features
- Access web apps from system
taskbar or dock
Prism apps run directly on your
desktop and can be accessed just like
any other application
- Rock solid web app stability
Prism apps run separately from the
browser, so they stay up even if your
browser doesn't
- System tray icon and dock badges
On Windows, the system tray icon can
be changed to give information about
application