How to permanently remove Growl from my Mac

A simple search finds http://growl.info/documentation/growl-package-removal.php from the growl official site. It also has this page http://growl.info/thirdpartyinstallations.php which may explain how it got on your machine. You should read it, or you may find it on your machine again.

Growl is just a notifier, you can turn it off in Preferences if you don't like it, or configure only specific apps to use it.

Go to the Growl Website download it, then run the uninstaller contained in the disk image.

i have found http://freemacsoft.net/AppCleaner/ to be particularly useful for all purposes.