Facebook Desktop Notifications adds a Facebook icon to your Mac's menu bar. Clicking on the icon lets you view any new notifications, the subject headings of messages in your inbox and gives you the option of updating your Facebook status right from your desktop.
Read on for screenshots of the app in action!
Additionally, the application supports Growl notifications, so can be alerted to any new activity via a semi-transparent pop-up menu.
You can also access your Newsfeed and other aspects of your account directly from the menu (it'll open up that page in your default web browser.)
The program is free -- and still in the experimental stages, so if crashes happen, don't be alarmed. Although the site says it was only tested for Mac OS X 10.5 Leopard, I've been running it on Mac OS X 10.6.1 Snow Leopard with absolutely no problems.
The app was developed by a Facebook employee (pay no attention to that "not developed by Facebook" message) and offers a nice way for Mac users to monitor their activity without having to always have a browser or Tweetdeck menu open.