http://www.stormwhistles.com/

These are very loud. If blown from a distance it can be heard without having to startle people up close. The extra distance also gives dog owners more time to reel in their pet. They work good in traffic too. They aren't accessed quite as quickly as a horn, but are less expensive, lighter, and don't require putting any extra junk on your bike. I keep mine on a key card recoil attached to my waist band.