Growl

http://growl.info/

User-Oriented Notification System for MacOS X (and Android, Windows, Linux, iOS)

Growl Network Transport Protocol (GNTP)
protocol to allow two-way communication between applications and centralized notification systems such as Growl for Mac OS X and to allow two-way communication between two machines running centralized notification systems for notification forwarding purposes.

GNTP Overview
An application MUST first REGISTER with the notification system. This registration will communicate the application's name, the notifications it may send, and details on those notifications. Requests to NOTIFY with information not corresponding to a previously registered application/notification pair will be ignored.

After REGISTERing, the application may at any time send NOTIFY requests. A NOTIFY request includes the information to be displayed, including title, text, and icon of the notification, and may also include hints to the display server, as detailed below.