Fluid is an application that allows you to create your own .app (application in your applications folder) from any website. It works great on sites such as GoogleReader or GoogleMail, as well as MobileMe and many other.
Fluid itself is a very small application. When launched, Fluid displays a little tiny window where you specify the URL of a WebApp you'd like to run in a Site Specific Browser. Provide an application name, specify a Location and an Icon (see flickr for lots of icons), click 'Create' and you'll be prompted to launch the new native Mac app you've just created.