Fluid is a really good app, but I can't see myself going over to a stack (like my Documents folder on the Dock) and opening it when I can just create a new tab in Safari. Is there any way I can modify Safari to force it to open the Fluid app when I enter a url that I created a Fluid app for?
Choosy is an app for doing exactly this -website here-. It lets you setup advanced behavior rules that can essentially say: "see this url configuration -> open it up in this browser" (see screenshot below). At first glance it looks like the rules pane is very configurable.
There is even a utility for it called (choosy fluid) Get it Here that will detect fluid instances on your mac and setup the appropriate advanced browser rules in Choosy so that the right links point you to the right fluid apps.
Don't forget to grab the browser plugin so that the rules you setup can be triggered from Safari.