Growl is a global notification system for Mac OS X. Any application can send a notification to Growl, which will display an attractive message on your screen.
a notification system for Mac OS X, allowing you to receive notifications from other applications without you having to switch from the current application.
A Mac OS X third party global alerting system. It is used by many Mac instant messengers to display events, such as when a contact signs in, a conversation starts, or nearly any action depending on the messenger.