Chin curtain, chin spoiler, chin skirt...usually it's used to direct the airflow under the helmet to eliminate turbulance and lift. Some companies also make it out of soft material and claim it keeps the wind noise down and keeps you warmer in the winter, but if that's true, it's by like .25 degrees F, and other helmet features do WAY more to help or hinder the wind noise than the little skirt does. It works to stabilize the helmet at speed, though. Any half-decent helmet should have one from the factory.
I don't know of any aftermarket companies that make them just for the chinbar, although there is a company that makes one that goes all the way around the helmet, and does a lot to remove wind noise and keep heat in during the winter...can't remember off hand what it's called though.