Nothing else that I've seen beats the airzound. The downside is that it generally doesn't work below about 35 degrees F. When I tried to use it that low it messed it up and it wouldn't work after it got warm either.

I took the back off, flattened the diaphragm, flipped it around the other way and it works again now. But still not in the cold. It'd be nice to find a diaphragm material that didn't mind the cold.

Other than that, they're all electronic and effectiveness is up for debate. The reviewers that love them seem to all say "I set this off inside and it was deafening." Yes, but inside and outside are COMPLETELY different things. Also electronic trills do not put fear into a motorist's heart like an air horn does.

The Hornit seems to be about the best.
http://www.amazon.com/dp/B006TDEV20

Or the Megahorn.
http://www.amazon.com/dp/B004IY7IJU

Both have people that like them.