Fluid is new app by Todd Ditchendorf to create stand-alone web apps that act like desktop apps, which are being called Site Specific Browsers (SSBs) by the creator. You can add your own icon, and it will run in it's own process, so a crash from another web page won't kick you out of your Basecamp or Google Docs session.
I've been using it for about a week since I heard about it on MacBreak Weekly #75: MacHeist Replies and a few other places, and I have to say I'm impressed by the simplicity of it. When you run it, it just asks for the URL, what you want to name it, where you want to put it, and if you want to use a custom icon, and you instantly have a new app. Fluid also has a javscript API so developers can communicate with the app to set Dock badges and trigger Growl notifications. It has this functionality built-in for a number of sites such as adding Dock badges for Gmail that display number of new messages.
I've just been using it for a dedicated Gmail app, and I really like having that separate from my other browsers and easily identifiable in the dock or when cmd-tabbing. Plus it feels more like an app without all the normal window chrome of a browser. Combined with the right custom icon, it's pretty convincing. All in all, I think this is a step in the right direction for web apps. I never much liked the Adobe AIR idea, since it's seems redundant to download another large runtime (15.5 MB) to make web sites cross-platform. You could argue the same about Fluid, but at 1.1 MB, it's pretty lightweight. And I think in the next year or two, this functionality will be built-in to browsers, so you can just do a "Save as App" to achieve the same thing. Mozilla is already working on a similar product to Fluid called Prism
Fluid is currently in beta at version 0.7 and is a free download though not open-source. You can download it here.
You can download the icon I'm using for Gmail in the screenshot above and bunch of others from the Fluid Icons pool on Flickr