Windows/Mac/Linux: Free application Photo Uploader for Facebook sounds like it's simply a tool for uploading pictures to the popular social networking site (which it does via a simple drag-and-drop), but it's also a robust desktop client for Facebook complete with notifications.
(Click the image above for a closer look.)
Facebook already offers their own desktop client—also built using Adobe AIR—but where their tool is rather limited, Adobe's Photo Uploader for Facebook is actually very functional. From Photo Uploader for Facebook, you can view your news feed, chat with friends, update your status, and upload photos by dragging and dropping them into the application.
My only complaint with this app is that I have a custom group of friends set up as my default home feed so I only see the updates I care about, but this app uses the default firehose news feed instead.
Photo Uploader for Facebook is a free download, works on any platform with Adobe AIR. If you're a die-hard Facebook user, it's a great app. If all you want is desktop notifications when something updates on Facebook and you're on a Mac, check out previously mentioned Facebook Notifications.