Mirrycle Incredibell Saturn Bicycle Bell
http://www.amazon.com/dp/B0017AVAME/...SIN=B0017AVAME

Available on Amazon.

I ride next to a noisy freeway on one side of a shared MUP and a busy road on the other that frequently has some peds on it. They don't respond to "on your left" because my voice will not cut through the road noise. For some reason, a good, strong bell noise makes them move. The Mirrycle is the best one I have ever used. It sounds great in all weather and is easy (no "thumb tab" to find). And people MOVE.