The QuickTime file format can be the cause of trouble in a production pipeline, with its moving-target gamma and all-or-nothing file integrity (if there's one corrupted frame in an .mov, forget about opening it and recovering the rest). QuickTime Player Pro, however, has many great features missing from other standalone players (including the fantastic VLC); it just requires that anything it opens be readable as a QuickTime movie. Although it seems at times to support other formats, they will often tend to open blank. Wouldn't it be great to just be able to double click any moving image file and open it in QuickTime?
Perian, the "Swiss army knife for QuickTime," is a free, open-source QuickTime component that more or less lets you do just that. The list of supported formats, video and audio types is a long one, but includes not only AVI but DIVX, FLV, MKV, GVI, VP6 and VFW. Change all of these formats to open with QuickTime Player (which the aforementioned Pathfinder can help you do smoothly) and all you have to do is double-click to open them in one place.
Not only that, but because the files are recognized as QuickTime they will also appear in applications that support QuickTime - not always without a hitch, of course, but with a high possibility of success (much higher nowadays than when Perian first appeard, for those who haven't checked in on it for a while). And if you're having trouble with a particular file QuickTime Player Pro even allows you to convert it to a plain MOV with your codec of choice.
The catch, if there is one, is that in order to work this alchemy Perian installs a plug-in at the system level and adds a Preference Pane. All such format extenders work this way, and Perian at least may obviate the need for alternatives such as Flip4Mac WMV and DiVX prefpanes. Although there is no evidence of system instability due to the installation of Perian according to Apple, they did warn of a specific bug that can crash Final Cut during AVCHD ingest. Perian is updated periodically and new version features are clearly described on its site.
As for the teaser in the title, to open and save a movie from YouTube in QuickTime, check out the second video on Perian's Watch page.
Note: For those running Windows, the commercial Flip4Mac does extend QuickTime's reach, but only to Windows Media formats.