http://www.youtube.com/watch?v=lPPuRx6CWVs&feature=player_embedded

personally I'd sit the guy up who caused the crash and kick them in the face

Oh wow, that was scary to watch. I expected the accident to happen at the hill, too. Dunno why the guy was driving in the wrong lane, though.

I would stand there filming the aftermath on my mobile phone.

Besides the obvious fault here, why did the guy he crashed into not move like all the others.
Clearly in your right mind you wouldn't play chicken with someone on the wrong side of the road (Also shouldn't be on the wrong side of the road in the first place)

I'd have went "Woooo" and continued on my way.

- typo

They should have called the authorities the moment they noticed him. (Perhaps they did.)

Anyway, priority would be to make sure the person in the other car is okay. If not, do what little you can to help before an ambulance comes by. Punching someone in the face at this stage wouldn't achieve anything. He's most likely injured himself, and is probably going to serve some time in jail. Appropriate time to punch him in the face would've been when he was getting into the car to begin with.