Flight controllers & stabilisers
These are little boxes of tricks that can do amazing things! The basic ones, such as the orange stabiliser, are a 3 axis gyro controlled stabiliser that makes corrections when the wind alters your aircraft's orientation, so that you get a much smoother flight. The expensive ones, wookong for example, will not only stabilise your aircraft but will also fly waypoints, return to launch, and GPS hold.
These functions are really useful if you want to fly fpv, especially the return to home function, as it is easy to get lost when flying fpv. Even if you are flying by line of sight and you lose your orientation, just flick a switch and the plane takes over and flies itself home. They also give you chance to fly on windier days when normally you'd be sat inside twiddling your thumbs!