/Library/QuickTime
. If you’ve got an older Mac OS X install and look in that folder, you’ll probably find all sorts of detritus from codecs of the past – DiVX.component, Perian.component, etc. These components are part of the old “QuickTime 7” framework. If you’re working in Final Cut Pro 7 or another older app, these are the only codecs that matter. This is also the only type of codec that can be developed by third parties./Library/Video
and /System/Library/Video
(And often in subfolders from there).