SocialBox is a nice Instant Messaging app for Facebook. It allows you to communicate with your Facebook contacts from your desktop, without needing to open another tab of your web browser just for that. The application was written in Adobe Air, which means that you can use SocialBox on Mac OS X, Windows and Linux. There are quite a few benefits in using SocialBox instead of the built-in chat used by Facebook these days.
First, SocialBox supports notifications. You can get a Growl-like notification whenever a contact gets online, offline, goes away, sends you a message, or comes back. These can be of course disabled from the Settings window, but it is great to have them. These notifications look like Growl notifications but they aren't, so you cannot really customize them. The second nice feature is support for video calls. Both users must be using SocialBox to access this feature, though. I tried it and I was surprised at how well it worked.
All in all, using SocialBox gives you more freedom on how you want to use the chat feature on Facebook. It gives you notifications, you can run it without a browser and it works very well. I might just use this from now on.