It's not free, but you can download the separate Growl library, which allows your application to display Growl notifications without having to install Growl on a user Mac.
See the Growl website for more information.
With that said, yes, the use of a growl is recommended because of what is familiar to him. However, if you intend to support Mac OS X 10.8+, I would say that just use the new Notification Center.
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