Mozilla has been working on a “Site Specific Browser” implementation called WebRunner that is a stripped down version of Firefox that allows you to connect to a particular site and it appears as an application to your desktop. It can have a desktop icon, it appears in Alt-Tab list by itself, and can be assigned an “icon” as well. This works great for website like Google Reader, or Google Calendar… providing you with a seperate instance for each that you can alt-tab to. You don’t need to go to your browser, and then switch tabs in the browser to see it.
External links open in your default browser, and with the minimal interface (basically just a status bar and the minimize, maximize and close buttons at the top… it gives as much space as possible to the web app you are using.
You can read more about it here: