What is the small fan I've seen on different turboprops?
This is the propeller synchronization (usually just called "prop sync") indicator, or "synchroscope." It rotates one way or the other to show the change in phase alignment of each propeller. During cruise, it's beneficial to manually or automatically (depending on aircraft) synchronize the frequency and often (again depending on aircraft) the phase of the blades to reduce noise and vibrarion (and consequently wear and tear).
In ancient times, before glass cockpits :), these were mechanical indicators with actual moving parts that rotated in response to the electrical signals from magnetic pickups mounted along the prop shaft.