Comment: I use

(See in situ)


I use

the DownloadHelper addon for firefox. It will download youtube videos and many others. Granted, there are some that it will not work with.
https://addons.mozilla.org/en-US/firefox/addon/video-downloa...

For command line use in Ubuntu (Linux), I use youtube-dl. It will even take a list to download a series of video's.
http://youtube-dl.org/