This one has been some sort of a favorite among dog trainers.
I even heard people talking about it when I was attending a dog exhibition. Its main qualities are its quite affordable price and its effectiveness that is best tested on somewhat short distances.
Still, I heard a trainer talking about calming his dog who was in next room just by pointing this device towards the wall and pressing a button. It turned out that FRIEDA dog repeller is also quite popular among joggers and cyclists because it prevents any hostile canine from approaching its prey.
Its sounds cannot be heard by humans and it is completely safe for dogs. Like most of the other products on this list, this device can also serve as a flashlight.
Affordable and effective on short distances, this small apparatus rightfully deserves the fourth place on this list.