I've got an Incredi-bell that is incredibly loud for a simple bell. I also have one of the Velo-Orange bells that mount on the headset, but the Incredi-bell is much louder. However, if you really one to get people's attention there is a horn called an Air-Zound or something similar that is supposedly like a car horn.

If you ever encounter joggers and road-walkers while cycling, a bell is essential. I don't use mine a lot, but it's kept me from potentially colliding with clueless joggers a few times.