Quackula said:
Also, what makes a good growl is really hard to pin down, it varies a lot from band to band and listener to listener. It's one of those things that's hard to define, but you know it when you hear a good one (assuming you're into that sort of thing, of course).
Some of my favourite growlers:
http://www.youtube.com/watch?v=gIXJFRVUdWo&feature=related