In Jersey we have *77, which is an anonymous number. You give the plate number and where the driver is headed. It's more or less for aggressive drivers, though, and I HAVE called it.
I felt that I needed to, because the driver seemed to be drunk or something-He kept swerving into the lane my boyfriend was driving in, almost hitting us a few times before my boyfriend slowed down drastically and we got his plate and a good view of the car to describe it.
I don't know how I could deal with myself if I had done nothing and heard later that he actually had hit someone and injured them, or worse.