youtube-dl

Installation from official site:

sudo curl -L https://yt-dl.org/downloads/latest/youtube-dl -o /usr/local/bin/youtube-dl
sudo chmod a+rx /usr/local/bin/youtube-dl

It's also on Ubuntu repos but you will often need to keep it up-to-date with sudo youtube-dl -U

Installation and ugrade from pip (/):

sudo -H pip install --upgrade youtube-dl

Usage:

youtube-dl YouTube-(or-other-website)-video-link

So it will download the video.

You can also take a look at this for very in-depth documentation on how to use it.

youtube-dl is a great tool for downloading YouTube videos. Install with:

sudo curl -L https://yt-dl.org/downloads/latest/youtube-dl -o /usr/local/bin/youtube-dl
sudo chmod a+rx /usr/local/bin/youtube-dl

Or with pip (python3-pip):

sudo -H pip install --upgrade youtube-dl

Or from Ubuntu's repo (older version)

sudo apt-get install youtube-dl

then use it this way:

youtube-dl [VIDEO_LINK]

For learning more about advance options, run

man youtube-dl

Or check out the documentation.

Original answer

For Firefox users:

You can install the Video DownloadHelper addon in Firefox and download any embedded object, including YouTube videos.

Minitube

Minitube is a YouTube desktop application. With it you can watch YouTube videos in a new way: you type a keyword, Minitube gives you an endless video stream. Minitube is not about cloning the original YouTube web interface, it aims to create a new TV-like experience.

It does not require Flash Player. From this application you can watch and download videos.

Browser addon:
http://www.clipconverter.cc/addon/

Other sites that can be used:

http://www.clipconverter.cc/
http://keepvid.com/
http://www.listentoyoutube.com/

And this software is also great:

http://www.4kdownload.com/

I think that the simplest way to download a video from YouTube, without installing any plugin or software, is with savefrom.net. And here is the video demonstrating How to download YouTube videos using the ss trick. Let's say you want to download the below video:

On your address bar, add ss in front of youtube.com and hit Enter ↵.

Now you will be redirected to savefrom.net. On your right, click on the video quality you want to download.

First, if you don't have wget, install it:

sudo apt-get install wget

Use this Perl script:

#!/usr/bin/perl -T

use strict;
use warnings;

#
##  Calomel.org  ,:,  Download YouTube videos and music using wget
##    Script Name : youtube_wget_video.pl
##    Version     : 0.38
##    Valid from  : March 2014
##    URL Page    : https://calomel.org/youtube_wget.html
##    OS Support  : Linux, Mac OSX, OpenBSD, FreeBSD or any system with perl
#                `:`
## Two arguments
##    $1 YouTube URL from the browser
##    $2 Prefix to the file name of the video (optional)
#

############  options  ##########################################

# Option: what file type do you want to download? The string is used to search
# in the YouTube URL so you can choose mp4, webm, avi or flv. mp4 seems to
# work on the most players like Android, iPod, iPad, iPhones, VLC media player
# and MPlayer.
my $fileType = "mp4";

# Option: what visual resolution or quality do you want to download? List
# multiple values just in case the highest quality video is not available, the
# script will look for the next resolution. You can choose "highres" for 4k,
# "hd1080" for 1080p, "hd720" for 720p, "itag=18" which means standard
# definition 640x380 and "itag=17" which is mobile resolution 144p (176x144).
# The script will always prefer to download the highest resolution video format
# from the list if available.
my $resolution = "hd720,itag=18";

# Option: How many times should the script retry the download if wget fails for
# any reason? Do not make this too high as a reoccurring error will just hit
# YouTube over and over again.
my $retryTimes = 5;

# Option: do you want the resolution of the video in the file name? zero(0) is
# no and one(1) is yes. This option simply puts "_hd1080.mp4" or similar at the
# end of the file name.
my $resolutionFilename = 0;

# Option: turn on DEBUG mode. Use this to reverse engineering this code if you are
# making changes or you are building your own YouTube download script.
my $DEBUG=0;

#################################################################

## Initialize retry loop and resolution variables
$ENV{PATH} = "/bin:/usr/bin:/usr/local/bin";
my $prefix = "";
my $retry = 1;
my $retryCounter = 0;
my $resFile = "unknown";
my $user_url = "";
my $user_prefix = "";

## Collect the URL from the command line argument
chomp($user_url = $ARGV[0]);
my $url = "$1" if ($user_url =~ m/^([a-zA-Z0-9\_\-\&\?\=\:\.\/]+)$/ or die "\nError: Illegal characters in YouTube URL\n\n" );

## Declare the user defined file name prefix if specified
if (defined($ARGV[1])) {
    chomp($user_prefix = $ARGV[1]);
    $prefix = "$1" if ($user_prefix =~ m/^([a-zA-Z0-9\_\-\.\ ]+)$/ or die "\nError: Illegal characters in filename prefix\n\n" );
}

## Retry getting the video if the script fails for any reason
while ( $retry != 0 && $retryCounter < $retryTimes ) {

    ## Download the html code from the YouTube page suppling the page title and the
    ## video URL. The page title will be used for the local video file name and the
    ## URL will be sanitized and passed to wget for the download.
    my $html = `wget -4Ncq -e convert-links=off --keep-session-cookies --save-cookies /dev/null --no-check-certificate "$url" -O-`  or die  "\nThere was a problem downloading the HTML file.\n\n";

    ## Format the title of the page to use as the file name
    my ($title) = $html =~ m/<title>(.+)<\/title>/si;
    $title =~ s/[^\w\d]+/_/g or die "\nError: we could not find the title of the HTML page. Check the URL.\n\n";
    $title =~ s/_youtube//ig;
    $title =~ s/^_//ig;
    $title = lc ($title);
    $title =~ s/_amp//ig;

    ## Collect the URL of the video from the HTML page
    my ($download) = $html =~ /"url_encoded_fmt_stream_map"(.*)/ig;

    # Print all of the separated strings in the HTML page
    print "\n$download\n\n" if ($DEBUG == 1);

    # This is where we look through the HTMl code and select the file type and
    # video quality.
    my @urls = split(',', $download);
    OUTERLOOP:
    foreach my $val (@urls) {
    #   print "\n$val\n\n";

        if ($val =~ /$fileType/) {
            my @res = split(',', $resolution);
            foreach my $ress (@res) {
                if ($val =~ /$ress/) {
                    print "\n\nGOOD\n\n" if ($DEBUG == 1);
                    print "$val\n" if ($DEBUG == 1);
                    $resFile = $ress;
                    $resFile = "sd640" if ( $ress =~ /itag=18/ );
                    $resFile = "mobil176" if ( $ress =~ /itag=17/ );
                    $download = $val;
                    last OUTERLOOP;
                }
            }
        }
    }

    ## Clean up the URL by translating unicode and removing unwanted strings
    $download =~ s/\:\ \"//;
    $download =~ s/%3A/:/g;
    $download =~ s/%2F/\//g;
    $download =~ s/%3F/\?/g;
    $download =~ s/%3D/\=/g;
    $download =~ s/%252C/%2C/g;
    $download =~ s/%26/\&/g;
    $download =~ s/sig=/signature=/g;
    $download =~ s/\\u0026/\&/g;
    $download =~ s/(type=[^&]+)//g;
    $download =~ s/(fallback_host=[^&]+)//g;
    $download =~ s/(quality=[^&]+)//g;

    ## Clean up the URL
    my ($youtubeurl) = $download =~ /(http?:.+)/;

    ## URL title additon
    my ($titleurl) = $html =~ m/<title>(.+)<\/title>/si;
    $titleurl =~ s/ - YouTube//ig;
    $titleurl =~ s/ /%20/ig;

    ## Combine the YouTube URL and title string
    $download = "$youtubeurl\&title=$titleurl";

    ## A bit more cleanup as YouTube
    $download =~ s/&+/&/g;
    $download =~ s/&itag=\d+&signature=/&signature=/g;

    ## Combine file variables into the full file name
    my $filename = "unknown";
    if ($resolutionFilename == 1) {
        $filename = "$prefix$title\_$resFile.$fileType";
    }
    else {
        $filename = "$prefix$title.$fileType";
    }

    ## Process check: Are we currently downloading this exact same video? Two of the
    ## same wget processes will overwrite themselves and corrupt the video.
    my $running = `ps auwww | grep [w]get | grep -c "$filename"`;
    print "\nNumber of the same wgets running: $running\n" if ($DEBUG == 1);
    if ($running >= 1)
    {
        print "\nAlready $running process, exiting." if ($DEBUG == 1);
        exit 0;
    };

    ## Print the long, sanitized YouTube URL for testing and debugging
    print "\n$download\n" if ($DEBUG == 1);

    ## Print the file name of the video collected from the web page title for us to see on the cli
    print "\n Download: $filename\n\n";

    ## Background the script. Use "ps" if you need to look for the process
    ## running or use "ls -al" to look at the file size and date.
    fork and exit;

    ## Download the video
    system("wget", "-4Ncq", "-e", "convert-links=off", "--load-cookies", "/dev/null", "--tries=10", "--timeout=20", "--no-check-certificate", "$download", "-O", "$filename");

    ## Print the error code of wget
    print "     error code: $?\n" if ($DEBUG == 1);

    ## Exit Status: Check if the file exists and we received the correct error code
    ## from system call. If the download experienced any problems the script will run again and try
    ## continue the download till the file is downloaded.

    if ($? == 0 && -e "$filename" && ! -z "$filename")
    {
        print " Finished: $filename\n\n" if ($DEBUG == 1);
        $retry = 0;
    }
    else
    {
        print STDERR "\n FAILED: $filename\n\n" if ($DEBUG == 1);
        $retry = 1;
        $retryCounter++;
        sleep $retryCounter;
    }

} # while

#### EOF #####

Copy this script to a .txt file and rename it youtube_wget.pl.

Make it executable, using the following command:

chmod 755 youtube_wget.pl

When you want to download in the terminal, type:

./youtube_wget.pl http://www.youtube.com/watch?v=ejkm5uGoxs4

Change the YouTube URL to your YouTube video URL.

Save Location: The video will be saved in your current directory.

Source: YouTube Downloader Wget Perl Script

cclive

Install cclive

We are able to download free content from YouTube or other streaming sites with this easy-to-use command-line tool.

cclive is primarily a "video download tool" but it can also be used along side with video player software, like "mplayer", for viewing streamed videos instead of the Adobe flash player.

To download a video from a given location (on YouTube press the "Share" button to get the URL to a video). Copy this link and issue clive from a terminal with e.g. the following command:

cclive -s best http://youtu.be/D6z6hn6wZlg

There are plenty of other options to control download behaviour including options to choose the download format that are all nicely documented in the manpage. We can also queue a list of URLs from a file.

(Changed -f to -s; -f is deprecated).

Available for Ubuntu 18.10 and older only.

For Firefox users:

As my answer to that question is probably more valid here, I'll present it here for convenience.

You can use flashgot, which is an addon for Firefox that I have found extremely useful for downloading videos from YouTube and other sites. (You still require the flash-plugin in your browser, however, for the video to be detected by flashgot.)

• Install flashgot from the official Mozilla addons site and restart Firefox. Make sure that the addon is enabled; check by going to menu Tools → Addons → Extensions. From the flashgot entry in the extensions tab you can also set a number of preferences.

• Navigate to your YouTube video and set the quality in the YouTube video settings and start the video playing for a few seconds and then pause it.

• Right click in the YouTube page and select flashgot media; there probably will be more than one video; either simply download them both or choose the larger file size (which will be the higher quality video). You can either use your browser's download manager or another program set via the flashgot options.

I want to suggest a method for downloading YouTube videos using VLC media player in simple four steps:

1. Open the link of the video as network streaming in VLC media player. (Ctrl + N or Media > Open Network Stream...)
2. Select media info and copy the location (Ctrl + I or Right click on the video area > Tools > Media information )
3. Paste it in your browser's address bar
4. Save it.

4kdownloader! Other than downloading videos from URLs, it can also download entire playlists, and also convert them to MP3 or other formats if required.

http://www.4kdownload.com/

I am using it for long time, it's simple and easy to use, and it has a GUI interface for Ubuntu.