Cameras and voice recorders  

Metro Aviation had been developing the IRIS system for years and installed it on all its platforms. It monitors everything the pilot and crew does. Do anything that is not considered an normal maneuver such as an approach with greater  than 500fpm rate or decent, or a bank in excess of 30 deg, you get a call from ops asking why.

All info is saved to memory on board for accident investigation. The Comm center can see a depiction of each aircrafts caution/warning panel in Shreveport and will call you on the radio if you get a light no matter where you are in the world. They have been using the HAA customers to improve the product with a view to selling it to the airlines and military.

Nifty stuff if you don't mind big brother looking over your shoulder. I worked for Metro a few years ago and am not sure what the status of the program is now.

https://www.metroaviation.com/2015/03/04/partnering-to-change-the-future-of-flight-data-monitoring-tracking-and-communications/