I know I have already covered this. But I think it is worthy of its own post; Just because it's so great.
Perian - Instead of trying to download every codex that Quicktime needs, download Perian. It is all of those codex (plural) and more. It is a native equivalent to VLC (which I never could get to work anyway).
For me, when I combine Perian and Quicktime Pro, I have a formidable video converting arsenal. I can download videos off of YouTube or any other site and convert them to any format I need. When I run across a format that won't play on a computer that isn't my Mac, I just bring it home and convert it to something that it can recognize.