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I just do it straight from Safari. Not the simplest way, but easy once you know what you’re doing, and no 3rd party software needed (although it downloads as a .flv file, so you’ll need something like Perian or VLC installed to play it)
 
I sometimes use GetTube, but you can easily find the .flv file you need to download if you open up 'activity' from the Safari 'window'-menu.
Just doubleclick that link and it starts to download 😀
 
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