Facebook has released a desktop application built with Adobe AIR. That means that it is platform independent (Mac, PC, Linux).
You will have to have Adobe AIR in your computer and then install the Facebook application.
Facebook also announced their Open Stream API technology. With the API now the limit is the sky and you will start seeing interaction of Facebook with unimaginable applications. For instance, I use brightkite, a sort of improved twitter with location and photos integrated. Well, now when I update my location, or I add a note or a photo in with my brighkite application in my iphone, it is published in my Facebook wall.
Facebook Desktop for AIR isn’t the only application to take advantage of the new APIs, but it is the first. It also has the benefit of being labeled an “official” application since it’s put out by the company itself. Still, many people will prefer more full-featured clients like Seesmic Desktop, for example, which also includes support for Twitter. The Seesmic folks say they’re working on a new app that will offer full Facebook Open Stream API support and it should arrive shortly.