Vixy.net (google vixy) is an online video converter that will convert online Flash video (FLV) into MOV (for QuickTime or iTunes), AVI (for Windows Media Player with DivX), MP4 (for iPod) or an audio MP3 format file.
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You may use Vixy to download MP3 audio or .mov videos from non YouTube video sites like blip.tv, Revver or Veoh provided the owner has shared content under an appropriate Creative Commons license. Vixy captures and converts the video clip in the browser itself from where you can download and save it to the desktop or your digital player directly.
While Vixy makes it possible to download YouTube videos to iTunes or convert YouTube vides to MP3 files, please abstain from doing so because Google Policies prohibit such an activity.
If you have a website, you can download Vixy Flash conversion engine since it is open source and offer video conversion or video to audio conversion services on your site for free.
Vixy also provides a desktop based software to convert FLV Flash videos to MPEG4 file formats like AVI, MOV, MP4, MP3 and 3GP for mobile phones. Unlike the Vixy online service, this software won't grab flash videos.
In my personal experience, Vixy.net changes the bit rate while converting a video to an MP3 song so the extracted sound or music is not always in sync with the video. You can however fix this in audio editing software like Adobe Audition or Audacity.
How To Do Everything with YouTube Videos
Update: In the case of Google Videos, you are permitted to save FLV Google videos as MP4 files for the iPod or PSP by clicking the "download video" button on the Google Video website itself.