There was notiMoo and I am sure there have been other javascript/mootools Growl notification solutions already. Yet, none of them were CSS3 and mootools 1.4 friendly, so I created a new Class instead.

Check out mooGrowl on GitHub
https://github.com/DimitarChristoff/mooGrowl

View the demo on jsfiddle:
Full screen or normal jsfiddle

Screenshot of some Growls: