I like these bells. Especially the Betty Page-style pinup devil with her bike on her shoulder.
http://www.somafab.com/hellsbells.html
I usually do a 3 part holler when I pass. "coming up behind you" "passing on your left" "thank you".
If you just shout "left" or "on your left" the pedestrians don't always know what you mean. (other bikers will) Walkers will often think you are telling them to move to the left, and they will do that and so end up right in front of you.
If/when I get a bell I'll use it for advanced warning before my hollering.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson