I just do it straight from Safari. Not the simplest way, but easy once you know what youre doing, and no 3rd party software needed (although it downloads as a .flv file, so youll 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