Mac Users: New QT component
 

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.

Try ffmpeg; it's free and can convert a lot of files types.
You can also try the new Flip4mac program. It is able t play more different types of media (particularly wmv), but make sure you don't set it as your default player when installing it.