Fluid lets you create an "app" out of a website. If you use one specific website all the time, this alone is very handy. For example, if you wanted a browser specifically for Gmail, you could make one using Fluid and set its own icon, download directory, and other settings. Or maybe you heard about Facebook tracking users even when they are "logged out" of Facebook and want to have a browser that you only use for Facebook.
Once you start making them, the uses for per-site browsers start appearing everywhere. I created one for my web hosting company's "web panel" with links on the bookmark bar to domain registration, support links, mail settings, etc. I don't need to see those links all the time, so I wouldn't want to put them on my regular browser's toolbar. But when I need to do something on my host, it keeps me from having to search all over their menus for the settings I use the most.
Is there a site that has a better iPad version than the regular version? Make a per-site browser for that site, and set the User Agent to the iPad from the menu. (Unlike Safari, that setting will stay even after the app is restarted.)
Fluid is free to use, but for $5 you get a few extra features:
Separate cookie storage (usually cookies are shared with Safari).
Minimize app to the menu bar instead of the dock
Userscripts or Userstyles
Lion Full Screen mode
Ok, honestly, #4 ought to be a default part of the app, but the others are really well worth the money. Do you use more than one Gmail account? With separate cookies, you can make separate browser-apps for each one, which makes it much easier. (Same goes for Facebook, Twitter, or any other social media site.) Separate cookies for Facebook means that Facebook can't track you whenever are doing when you aren't using Facebook. Worried about Google invading your privacy? Create a Fluid.app for Google and keep its settings separate from your other sites too.
Do you like to listen to Internet radio? Make a browser that only appears in the menu-bar and you can easily control it without cluttering up your dock. Use Google Calendar? Create a browser for it, set the User Agent to "iPhone" and put it in your menu bar for quick reference.
Using Fluid.app Browsers With 1Password
Unfortunately, Fluid.app browsers do not integrate with 1Password. I'm hoping that might change in the future, but in the meantime, the good news is that you can still use your 1Password data using a feature called 1PasswordAnywhere. The short version is this
Locate your "1Password.agilekeychain" file
Control-click » Show Package Contents
Find 1Password.html file
(If your 1Password file is in Dropbox, the path will be ~/Dropbox/1Password/1Password.agilekeychain/1Password.html.)
Put the full, complete path into your Fluid browser. For example, mine is:
(Be sure to change /Users/luomat/ to your path.)
That way you can open your 1Password file (read-only) in your Fluid browser. (Note: if you have restricted what sites your Fluid browser can access, you may need to loosen that to allow it to access that page.)
The only other drawback is that Safari extensions don't work with Fluid browsers. Also, you may have heard that Google Chrome has a similar feature called "Application Shortcuts." Unfortunately, that feature is not available on Mac OS X, and Google has not said if or when it ever will be.
In a world where "web apps" are becoming more and more common, Fluid makes them easier to use than ever. You can use the majority of features for free, so go ahead and download Fluid and check it out. It requires 10.6 or later. Once you start using it, you'll probably find even more uses for it. Some day I'll explain my byzantine system of using Fluid and Choosy, but in the meantime, take it for a spin.