How To Download YouTube Videos with MPEG Streamclip.
Grab the BETA version of MPEG Streamclip.
OPEN MPEG Streamclip.
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From the FILE menu, Choose OPEN URL. This dialog box pops up:
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In your browser, navigate to the YouTube Video you want to download.
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Grab it's URL.
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PASTE that URL into the Open URL dialog box on MPEG Streamclip.
Select the Download and HD radio buttons and CLICK Download.
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Enter a save location and click SAVE.
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Your video is now being downloaded.
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Click it open and there it is!
via [ Robin Good ] who linked to it on [ The Editblog ] where there are lots more options explained.