I've never heard of this before but Perian
http://perian.org/ is an all in one quicktime component that allows you to play a whole host of different video formats in QT player, from Xvid to FLV.
Kinda useful but I still have some .mpg files encoded with the Microsoft mp43 codec that just won't play in QT no matter what I try. I can't get any of the perfect gonzo files to play either.