The only one I've installed so far, and it's a definite keeper, is "Download Media Bar".
I've tried quite a few video downloaders before this one, and they all caused serious flickering graphics glitches when you activated them.
This one works smoothly and faultlessly and works with a lot of different video sites.
If you try it, it's quite hard to spot the download buttons, but they become visible when you mouse over them: