Came across them recently and decided to give them a try. Fluid (3rd party) and Prism (Mozilla) are applications that let you run a URL in its own window on your desktop. Perfect for webapps like Gmail, or any website you have open constantly.

Fluid has the ability to minimise your webpage to the Mac's menubar and I've set up my Gmail like this:

[IMG]http://kttns.org/nznmz[/IMG]

Pretty wicked. Also cool that they run independently of Firefox which I hope will lessen the browser's memory footprint.