Facebook Desktop, an Adobe AIR app links into your Facebook account and notifies you of every change to your account -- pokes, messages, friend requests, event invites, etc. -- with a little pop-up toast window on your mac desktop. The notifications appear in the bottom right of your screen whether you like it or not -- you can't change it to another corner, and you can't change their appearance.
Beyond notifications, it also lets you publish status updates -- but as you've to go via a tiny system tray icon, and there's no way to set a global hotkey, it's probably easier to just pop open a browser window and update your status normally.
Note, it's a 45MB memory footprint, and requests a truly insane number of permissions when connecting to your Facebook account:
More Info: Facebook Desktop
[tags]snow leopard,desktop,laptop,social media,social networking[/tags]