Bells

Get pedestrian and bicyclist attention with this thumb bell

I think bells sound friendlier than a loud air horn, as do some of the sillier squeeze horns.

The B-R-RING - B-R-RING sound of a thumb-lever-activated bell instantly and unmistakably identifies you as a bicyclist and often gives away your general position. This might be the best kind of signaling device to get.

In any case, it helps to smile at someone you've signaled. This might work better than looking at them with an expression that says, "Get out of my way!".

Unfortunately, motorists will sometimes be unable to hear these devices.