I have to say Growl is pretty indispensible to me. It's being used by Adium, Transmission, Cyberduck, pretty much everything. It still doesn't work with Mail in Leopard (no matter what anyone says and what I try, including the method mentioned previously in this thread) which is a nuisance because that was incredibly useful.
I can't imagine my mac without Growl. It's just one of those little things that make it a pleasure to use, along with Quicksilver.