Given a YouTube or Vimeo URL, downloads the underlying video file, with a sensible file name. It downloads the highest resolution version of the video available: first it tries HD MP4, then regular MP4, then WebM, and finally FLV. It also works on playlists, and works as a bookmarklet[doesn't exist] to download the video you're watching.
Tested 2016-03-18 on Lubuntu.
For higher quality stuff, it begs for ffmpeg.
\sudo \apt-get install ffmpeg