A bell is a useful accessory for almost any bike, especially if a lot of your riding is urban /suburban. - Of course, they don't work very well for pedestrians with their i-pods clogging their ear canals. (Anyone offering air-horns for bikes?)
Anyway, there is quite a variety of bike bells, some very plain, others very novel, available over a wide range of prices.
I've purchased a number of these cheap generic colored bells on eBay (below): I like the colors, but I have to say they are prone to breakage.
I've also purchased a few
Incredibell brass duet bells from the LBS. These have a nice double tone, and are more durable. (I'd recommend them over the cheap ones.)
What kind of bell do you use/recommend?