Minitube, in the repositories, is good both as a viewer and a saver of
Youtube videos.

Minitube is great. I really like it when you type in something like "rifleman"
and it brings up videos that are available without having to google it.

I use 'DownloadHelper' Extension for Firefox, works good for youtube etc.

This Firefox extension is great for downloading youtube or any other videos. I
like the fact that you can choose the size of the file to be
downloaded.