Right-click onto a link to a YouTube video or at the background of the YouTube
site showing the video and choose action -> Download YouTube Video -> ...

Automatic Installation (may not work):
tar xvjf youtube-servicemenu*.tar.bz2
sudo ./youtube-servicemenu*/

Manuall Installation (not tested):
tar xvjf youtube-servicemenu*.tar.bz2
cd youtube-servicemenu*/
sudo cp /usr/bin/
sudo cp get_yt_video.desktop /usr/share/apps/konqueror/servicemenus/get_yt_video.desktop
sudo mkdir -p /usr/lib/get_yt_video/
sudo find l10n -mindepth 1 -maxdepth 1 -type d -exec cp -r {} /usr/lib/get_yt_video/ \;

I made some modifications to the YouTube Service Menu. It didn't work for me,
so I looked at the source. I added a different method to guess the video_id and
I added additional parameters to the query string to download the video.

With this konqueror service menu it is possible to download a youtube video
by right clicking a link to the video and choosing "get youtube video" from the
actions menu.

For technical details see:

See also the original version(s) of this script: