Theres a lil electronic doohickey that you can buy, fits up behind your rearview mirror with a cam pointing out the windscreenand continully records onto built in memory, deleting the "begining" as it fills the memory, it has a button it that you can press and it automatically saves the last 30 seconds it recorded and the next 30.
The idea is if someone cuts you up, you hit the button and it should show the incident happening which you then download and can then use as evidence if you want to report it, it does the same thing if you have a crash as well which could be handy, only trouble is i cant remember what its called and cant find it on the net. :S