Fluid is now solid. Developer Todd Ditchendorf has, after three and a half years, released Fluid 1.0, the first non-beta release of his software for creating single-site browsers. It's also, not inconsequentially, a brand new, completely fresh version of the app, rewritten from scratch.
With Fluid, you can create a true, standalone Mac app devoted to accessing a single Website. I've used Fluid for years to create Mac apps for accessing Gmail, my Google Calendar, and other sites (even, for a brief period about which I now feel great shame, Facebook). Fluid is smart enough to update badge icons for certain sites, so that the Dock icon for your Gmail Fluid app can reflect the number of unread e-mails in your inbox.
Fluid is free, but a new $5 Fluid License gets you access to several extras. The one I'm most excited about is the app's new option for Separate Cookie Storage. Previously, Fluid--which really creates custom-wrapped WebKit views for the apps you create--shared Safari's cookie repository. With this new feature, you can sandbox every Fluid app you run. That means you could, for example, create a standalone Fluid app for Twitter, in which you could remain logged into a separate account from the one you access via Safari.
The license also grants the ability to turn Fluid apps into menubar-only utilities (which I suggest doing at the end of Step 5 in Macworld's guide to switching to Google Calendar), along with the option to use Userscripts or Userstyles in your Fluid apps. Userscripts let you create custom AppleScript triggers for your app without any hackery, and Userstyles make it possibly to override a site's stylesheets with your own display options.
If you don't need those extras, though, then Fluid's core features are free. The app requires Mac OS X 10.6 Snow Leopard or later.