Fluid lets you create what they refer to as "site specific browsers" – basically a website running in it's own window, acting more like a standard OSX application.
I was introduced to Fluid by a friend the day after I picked up my Mac – the whole "site specific browser" thing is an idea which is so simple and obvious that you wonder why it took so long to create. I've been using Fluid for months, but hadn't bothered to look at a couple of the options which can make it even more powerful.
This video is a nice summary of what Fluid is, and what it can do. The transparency options and menu bar items are especially handy for certain applications.