A classic from Rivendell:
Electrabike has many options including this cheery yellow:
Perhaps a ladybug for good luck? From WishingFish:
Blingarific from Abbey Hill Creations:
A very popular choice available in my LBS (local bike store) is this Jelly Bell (more colors are available):
And finally, my simple silver and black bell is perhaps the simplest of the many bells offered by Mirrycle: