During the February 1st, 2010 Mac Geek Gab Podcast cool stuff found segment, John and Dave talked about Perian.  Perian is the self proclaimed Swiss-Army Knife for Quick Time.  As the site states:

“Perian is a free, open source QuickTime component that adds native support for many popular video formats.”

As a OS X user I get emails from family members with video, that most often will not play in Quick Time, until no. Previously I had been using the plugin, Flip4Mac, but a small number of video files still failed to play.  After installing Perian, many file formats became available via Quick Time. The best part about Perian is that it is free and is compatible with Quick Time and Quick Time X.