We've covered the concept of site specific browsers before. Basically these are stand-alone browsers designed to work with just one website, e.g. Gmail. Fluid takes this idea to the next level by automating the process of creating Webkit based site specific browsers. When you launch Fluid it asks for the URL and name of the site, and then it creates a Cocoa application just for that particular site. The reason that this is handy is that it separates your general browsing (and related crashes) from your Gmail, facebook, etc.
Fluid is in beta and can be downloaded from Todd Ditchendorf and requires Leopard.