Growl is a notification system that receives and displays messages sent by applications. You can (for example) be notified that you have new email, or that a download has completed.
Easily update apps from App Store and other sources.
OmniGrowl provides many practical notifications through Growl in a single application.
Mac Live can provide growl alerts when selected friends come online on xbox.com.
Google Notifier is Google's application for notifying you on new Gmail messages.
An application to notify Growl or Prowl of login or log out of the Mac OS X.