Video cameras
As a non-flyer I hesitate to intrude into such a thread, but every time I see reports of an aircraft accident and the resulting confusion I wonder why cameras are not routinely installed.
I get 1 hour of 640*480 video on a 2gb memory card so surely the cost of recording is not a major problem.
You could probably cover all runways in an airport in good detail with about 20 cameras.
As the aircraft tailplanes seem to survive most accidents, perhaps cameras could also be mounted there which would show engine condition, flap and thrust reverser deployment, etc.
Apologies if this causes any offence to other posters.