youtube-dl is probably the most popular command line YouTube (and not only) downloader for Linux but there’s a new tool that tries to do things a bit differently.
mps-youtube is a new tool which, besides being able to download YouTube videos (either the whole video or just the audio), can also search and play YouTube videos and create local playlists, all from the command line.
By default, this is basically a YouTube audio player (and downloader), but you can enable (external) video playback from its options.