Fluid is an application you can use to create Site Specific Browsers (SSBs) on the Mac based on the Safari rendering engine (WebKit). The author freely admits that the concept was inspired by Mozilla Prism, which does something similar for Firefox and its rendering engine, Gecko. As well as Adobe AIR which brings its own tricks to the party beyond just duplicating a browser.
The beauty of a browser like Google Chrome is that each tab runs in its own space in memory. If a web app misbehaves, you can shut down that tab and free up its resources without bringing down the whole browser. Fluid brings a similar concept to the Mac in an oh-so-elegant way that relieves most of the pain I had in switching from Firefox to Safari.
As a matter of fact, I tried the latest Firefox release candidate the other day and found that I still prefer Safari for day-to-day use. Fluid-created SSBs make the experience near perfect for me.