Messenger for Mac is a standalone app that transforms the new Facebook messenger interface. This is an extremely speedy example of app development, since this new interface was released three hours before the app was created.
It's ideal if you want to converse via Facebook message without getting distracted by the actual social media platform (students studying for exams, I'm looking at you). You can simply go to messenger.com, or if you have OS X, use the dedicated Messenger for Mac app.
Described by Tech Crunch as "essentially a webpage wrapped in a sandboxed browser," the project was designed by Dropboxers Rasmus Andersson and Josh Puckett. This means the source code is available to peruse and modify on GitHub.