Daniel K. Bergey is the original creator of the display style aptly named NotifyOSX for Growl. I have been a fan of both Growl and NotifyOSX for quite a while. Growl is basically a framework (a way for applications to easily tell other applications to do something, for Growl; display something). In the same way as Daniel I use it to display various notifications; including incoming Mail, finished transfers, what track iTunes is playing, when I receive chat messages from friends and so on. It is very convenient.