Car cameras

Hi all, was wondering if anyone has had any experience with cameras fitted to a car which records things going on around it, a bit like a black box for car. Reason being because it'd be useful for incidents where my car gets dented by inconsiderate people who don't know how to open doors when parked next to me, or for a car accident that might be hard to prove who was in the right.

Basically, I have no clue about what I should be looking for and if someone can point me in the right direction, I'd be all set.