It's called a "pole breaker".
Some people call them fuses, but they are resettable.
Here's a pic from the web showing one that has opened due to high current, or a fault on the line.
(Or, maybe the utility is working on the drop and have it open for a reason?)
http://waterheatertimer.org/images/Tripped-fuse-IMG_2699-1000a.jpg
It's the lower portion of the device, located at center screen, right side of the insulator, which itself is on the right side of the pole.
(I'm too lazy to annotate it with an arrow - but it should be very obvious.)
The hook you see on the left-hand side of YOUR image is where the utility worker attaches a "hot-stick", which is a special insulated pole for working on "live" high-voltage power lines.
These are very common. Now that you know what it is, you'll see them everywhere on power poles in your neighborhood. (typically the poles in residential areas - these are not used on the big long distance transmission towers).