Fluid lets you run any webapp as a separate Mac desktop application if you've got Leopard.
Are you a Gmail, Facebook, Campfire or (Insert Your Favorite Webapp Here) fanatic? Do you have 20 or more browser tabs open at all times? Are you tired of some random site or Flash ad crashing your browser and causing you to lose your (say) Google Spreadsheets data in another tab?
If so, Site Specific Browsers (SSBs) provide a great solution for your webapp woes. Using Fluid, you can create SSBs to run each of your favorite webapps as a separate desktop application. Fluid gives any webapp a home on your Mac OS X desktop complete with Dock icon, standard menu bar, and logical separation from your other web browsing activity...
How does it work?
Fluid itself is a very small application. When launched, Fluid displays a small window where you specify the URL of a webapp you'd like to run in a Site Specific Browser. Provide an application name, click 'Create' and you'll be prompted to launch the new native Mac app you've just created.
Use Fluid to run YouTube, GTalk, Flickr, Basecamp, Delicious, .Mac webmail, or any other webapp as a separate Mac desktop application.
(If you're not on OS X, Prism does something similar for Windows and Linux powered by the Mozilla engine).