http://omz-software.com/vidl/

ViDL is a free Mac app that allows you to easily download videos from YouTube and hundreds of other websites for offline viewing.
It is based on the popular youtube-dl command line tool, but much easier to use, especially with videos/playlists that require a login (like your personal "Watch Later" list).

Man kann sich ein Bookmarklet in die Browser Favoritenleiste legen und dann per Klick das Video runterladen lassen.