I was thinking about this recently; seems like affordable consumer quadcopters are a consequence of the availability of cheap, lightweight solid-state accelerometers. Why are accelerometer chips cheap? Because they're ubiquitous in phones. Why are they ubiquitous in phones? Because Steve Jobs wanted the original iPhone to be able to tell which way up you were holding it.

(also, due credit to laptop hard drive 'freefall sensors' as another contributor).

Combine that with the cheap lithium battery and digital radio communication chipsets, also driven by phone volumes, and that's why 'radio controlled helicopters' are no longer a niche hobby, and instead a danger to aviation.