SaveFrom.net. It's not the website that'll surprise you, but the way you download videos that is so unique and useful.
You can take any video, say this DramaAlert video that was on my front page-
and just add ss after the www. and before the youtube.com
https://www.ssyoutube.com/watch?v=si3lQH_iCT0
And there you go! You have a working download link. Just click the small blue "Download Video in Browser" text on the right hand side, and a popup asking you where to save it will appear.