Notification system that allows any application to send it a notification, and then it displays it.
Growl is a notification system that allows any application to send it a notification, and then it displays it. You can (for example) be notified that you have new email, or that a download completed.
This tool may not be the most essential one, but it can make the work process more comfortable, for sure. It is more convenient to receive all notifications in one place.