Fluid is an app to (kind of) create apps:
Fluid lets you create a Real Mac App (or "Fluid App") out of any website or web application, effectively turning your favorite web apps into OS X desktop apps.
Facebook, Pandora, Gmail -- any app you use in a browser tab you can turn into a standalone application.
Why is this useful? For me, it lets me have my YNAB app in its own browser window so that I can Command-Tab quickly to the browser to view my credit union accounts page and then back to the YNAB display.
Another example: I'm writing this post in a SquareSpace "fluidapp." I can quickly Command-Tab to the browser to do quick searches or copy URLs without doubling back to hunt among my tabs for the blog posting window.
Yes, sure, I could Ctrl-Tab to troop through the browser tabs, or I could simply open the browser tab in its own browser window.
But I like the cleanliness and simplicity of having some web apps running in their own application windows. It means much less friction for me overall.