i don't have one. i do sometimes feel the need.
i have one of those honky horns, but i haven't got the 'shameless factor' to use it!
i've shouted 'idiot' a couple of times to great effect, but i think its time i got a bell of some sort as i often break my pace due to 'idiots' with no awareness of their surroundings causing me to brake - but if i;d tinkled my bell, they would have been given enough warning..
being in a 'bicycle friendly city' it still astounds me how many people simply don't seem to realize when they are walking in a cycle lane - despite its bright red colour and when crossing over turnings, its often a case of 'oh well if i can't hear a car coming, its safe to cross' type attitude. lots of near misses - the 'missing' part down to me being aware of their stupidity.