Most of the time, when I'm out walking a dog the dog will get spooked by the following - fire hydrants, guard rails, for sale signs stuck in the ground, moving vans, water puddles, manholes (are they still called manholes?), random children's toys. Especially if it's dark out and the objects are reflecting light. All I do is go near the object and stand still. The dog investigates at his own pace, and when he's comfortable with it we move on.
I have brought several dogs to work with me, there is a lot of machinery in the back that is frightening to a dog at first. So I've had to walk my dogs through the warehouse so they could see where the noises were coming from, then they were fine.
A few more - The ice cream truck, the mail truck, any delivery truck. And of course the school bus. There is tarp hanging on the chain link fence surrounding the baseball field next to my dog park. That has caused a couple of my foster dogs to go insane when it flaps in the wind. It makes a loud crashing noise.
I had a dog once who would lie on her belly and not move if we went passed a fence where a dog was barking on the other side. I was only temp fostering this dog as she was slated to go to a foster home with more experience with anxious dogs. So I never got to find a solution with her. She walked passed one barking dog behind a fence before I took her to her new home, but I still am not sure if it was someting I did that helped her or not.