When it's done, Friends.app will load your Habbo friends feed and give you desktop notifications of who signs on, and off; without the need to have the game open. You will also be able to access your friends from the menubar item. Pro users will be able to enjoy fullscreen play.
Currently all that's usable right now is fullscreen play, which is free for everyone! Please let me know if something breaks, my email is in the About window.