Many YouTube to iPod/Quicktime format converters exist, but most either cost money or are impractical. As an alternative,
vixy.net is a free, online, fast video conversion tool. Export YouTube videos to a variety of formats, including iPod, QuickTime, and AVI. A frame of a logo is placed at the end of the movie, but other than that, there is no video alteration.
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robg adds: I tried this with a 2:48 video from YouTube, and it worked -- just don't try to use the Save button on the site. I first tried to click Save, and got a "400" error. Then I noticed that my browser (Camino, but Safari does the same) had auto-downloaded the file.
The conversion isn't the quickest, particularly because the site can't tell your browser to maximize CPU usage, so things happen based on the browser's priority (open mouth insert foot!). On my Mac Pro, it took about six minutes to convert that 2:48 clip. The end result, though, seems to be of very good quality (obviously tied to the quality of the source).]