have the same bell on my commuter that redthunder posted (I love my bike..)
great for riding in the city (London), seems much more polite than shouting, many times people have said “thank you” after I have dinged my bell to warn them I am coming through
but of course from time to time, I have to shout as the white headphones (I-Pod) mean the pedestrian cannot hear my repeated bell dinging and go to step off the pavement and end under my wheels!
don’t have one on my MTB because I ride in an area with few people ever seen
and to clarify, British Standards dictate a bell must be fitted to every bike I sell through my workshop, but after purchase the customer is able to remove their bell as the Highway Code suggests a bell as a “recommendation” rather than a requirement for running a road legal bicycle