If you wish to download online videos from sites such as YouTube or Google Video, you may be dismayed to discover that you are unable to view them in QuickTime. Unfortunately, Quicktime does not support FLV files at this time. However, with a combination of Perian and an additional plug-in (flv.glgenerator), you can easily view FLV files in QuickTime.
You can obtain the plug-in file from: http://homepage.mac.com/xdd/software/flv/ . The Web page is not in English but you can download the plug-in by clicking on the flv.qlgenerator link. You will then need to copy the flv.glgenerator file to the /Library/QuickLook/ folder under the Macintosh HD folder (not your user folder).