The latest feature from Facebook is Prototypes. Similar to other beta testing services i.e. Google Labs, Mozilla Test Pilot, Facebook Prototypes give users a chance to test out cool new applications and functionality "going on inside the walls of Facebook" and to provide feedback to the who built the prototypes, directly.

To begin testing and providing feedback, simply find any application available in the Application Directory that is marked as "Prototype". Some cool examples that have come out of Facebook prototype apps include Desktop Notifications for Mac, recent comments filters and enhanced event emails. [via]