Perian

Perian is an amazing component for Mac OSX quicktime that allows Quicktime to handle additional video codecs, including Flash video. If you want to use flash clips with the also-incredibly-useful MPEG Streamclip, (say to trim and convert videos downloaded from youtube) Perian will allow you to use them. It's just become one of those mandatory to install programs for me. I used it recently to convert some public domain video on Youtube to DVD and DV.