My old broken bike has a honk-honk horn that my wife gave me. I've never used it in anger, it's generally used by people at traffic lights, because they can't resist walking up and giving it a good squeeze.
I find bells aggressive, I'm not sure why. I think in part it's a cultural thing – we fear the bicycle – and in the same way, car horns in the UK seem aggressive. I learned to drive in NYC and you lean on the horn all the time, no one generally gets too het up if you honk them, it's a general hey-I'm-here.