Growl is a global notification system for Mac OS X. Any application can send a notification to Growl using General/NSConnection, General/NSDistributedNotificationCenter, General/CFDistributedNotificationCenter, General/AppleScript, or UDP. Growl will then display an attractive message on your screen, or send you an email, or log to a file - it’s up to you.
Growl works with a growing number of applications. There is a list here: http://growl.info/applications.php
Growl can be used by many languages. Some of these languages require bindings, and others a framework. Both the bindings and Frameworks can be found in the SDK: http://growl.info/downloads_developers.php (most Cocoa and Carbon developers tend to use the Framework provided). An example of the many languages which are supported include: Cocoa, Carbon, Perl, Python, Ruby, Java, Applescript and TCL.
Growl’s website: http://growl.info